feat: Enable use of latest: in .tool-versions files

This change enables `asdf`'s existing latest-version-resolution
functionality within the `.tool-versions` file itself. Rather than
having to have a `.tool-versions` file that contains a full version
number:

```
java corretto-21.0.5.11.1
```

...you can now use the same `latest:` syntax that is already available
in the `local` & `global` commands, ie:

```
java latest:corretto-21
```

### Use case

For many tool/runtime ecosystems (eg Java), if a program runs correctly under
a specific version of that runtime, it can generally be relied on to run
correctly under any _later_ version of that runtime with the same major version
number (eg if a project runs under Corretto Java 21.0.5.11.1, it will run on
any _later_ version of Corretto Java 21).

This means that for projects in those ecosystems, there is little incentive
to pin to fully-specified versions like `21.0.5.11.1`, and in fact there are
downsides - over time, developers will default to using older, unpatched versions
of Java, unless they are assiduous in continually updating the contents of
the `.tool-versions` file, or have tooling devoted to doing so.

At the Guardian we have several hundred projects that run on the Java platform,
and due to our security obligations we generally want to be running under the
_latest_ security-patched version of the Java runtime that matches our
major-version requirement. We love `asdf` as a tool, and like that the
`.tool-versions` file can become a source-of-truth documenting which version
of Java a project uses, but we don't want to have to commit fully-specified
version numbers like `21.0.5.11.1` to source control, or set up tooling to
increment those version numbers across those hundreds of repositories.

Allowing the use of `latest:` in the `.tool-versions` file means that we
don't need to continually update those `.tool-versions` files. It also
partially addresses some of the needs raised by asdf-vm#1736,
though this solution uses the existing `asdf` version-resolution functionality,
rather than adopting the version requirements system used in nodejs.

### Implementation

A new `resolve_version_spec()` function has been extracted from the
existing `version_command()` function. This takes a version-spec string,
like `latest:corretto-11` or `corretto-21.0.5.11.1`, and resolves it to
a definite installed version number (if the resolved version is not
installed, the appropriate error message is shown).

This new `resolve_version_spec()` function is now called in
`select_version()`, used by the `with_shim_executable()` function,
meaning that any execution of the `asdf` shim (eg, executing `java`)
will now resolve any version specifications found in the `.tool-versions`
file - if `.tool-versions` contains `java latest:corretto-21`, this
will be resolved and the latest version of Java 21 used.

## Other Information

Previous `asdf` PRs relating to `latest`:

* asdf-vm#575 in November 2019: added the `latest`
  command, eg `asdf latest python 3.6` reports the latest version of Python 3.6.
* asdf-vm#633 in July 2021: made it possible
  to specify `latest` when using the `local` & `global` commands, eg:
  `asdf local python latest:3.7` - this would save a precise version number
  to `.tools-versions`, which is undesired behaviour for us at the Guardian.

A couple of Guardian systems attempting to standardise on using `.tool-versions`
as a source of truth:

* guardian/gha-scala-library-release-workflow#36
* https://github.com/guardian/setup-scala

bin/asdf

@@ -7,6 +7,8 @@ fi
 
 # shellcheck source=lib/utils.bash
 . "$(dirname "$(dirname "$0")")/lib/utils.bash"
+# shellcheck source=lib/functions/versions.bash
+. "$(dirname "$(dirname "$0")")/lib/functions/versions.bash"
 
 find_cmd() {
   local cmd_dir="$1"

lib/functions/versions.bash

@@ -37,27 +37,7 @@ version_command() {
   declare -a resolved_versions
   local item
   for item in "${!versions[@]}"; do
-    IFS=':' read -r -a version_info <<<"${versions[$item]}"
-    if [ "${version_info[0]}" = "latest" ] && [ -n "${version_info[1]}" ]; then
-      version=$(latest_command "$plugin_name" "${version_info[1]}")
-    elif [ "${version_info[0]}" = "latest" ] && [ -z "${version_info[1]}" ]; then
-      version=$(latest_command "$plugin_name")
-    else
-      # if branch handles ref: || path: || normal versions
-      version="${versions[$item]}"
-    fi
-
-    # check_if_version_exists should probably handle if either param is empty string
-    if [ -z "$version" ]; then
-      exit 1
-    fi
-
-    if ! (check_if_version_exists "$plugin_name" "$version"); then
-      version_not_installed_text "$plugin_name" "$version" 1>&2
-      exit 1
-    fi
-
-    resolved_versions+=("$version")
+    resolved_versions+=("$(resolve_version_spec "${versions[$item]}")")
   done
 
   if [ -f "$file" ] && grep -q "^$plugin_name " "$file"; then
@@ -79,6 +59,32 @@ version_command() {
   fi
 }
 
+resolve_version_spec() {
+  local version_spec=$1
+
+  IFS=':' read -r -a version_info <<<"$version_spec"
+  if [ "${version_info[0]}" = "latest" ] && [ -n "${version_info[1]}" ]; then
+    version=$(latest_command "$plugin_name" "${version_info[1]}")
+  elif [ "${version_info[0]}" = "latest" ] && [ -z "${version_info[1]}" ]; then
+    version=$(latest_command "$plugin_name")
+  else
+    # if branch handles ref: || path: || normal versions
+    version="${versions[$item]}"
+  fi
+
+  # check_if_version_exists should probably handle if either param is empty string
+  if [ -z "$version" ]; then
+    exit 1
+  fi
+
+  if ! (check_if_version_exists "$plugin_name" "$version"); then
+    version_not_installed_text "$plugin_name" "$version" 1>&2
+    exit 1
+  fi
+
+  printf "%s\n" "$version"
+}
+
 list_all_command() {
   local plugin_name=$1
   local query=$2

lib/utils.bash

@@ -736,7 +736,8 @@ select_version() {
     version_and_path=$(find_versions "$plugin_name" "$search_path")
     IFS='|' read -r version_string _path <<<"$version_and_path"
     IFS=' ' read -r -a usable_plugin_versions <<<"$version_string"
-    for plugin_version in "${usable_plugin_versions[@]}"; do
+    for version_spec in "${usable_plugin_versions[@]}"; do
+      plugin_version="$(resolve_version_spec "$version_spec")"
       for plugin_and_version in "${shim_versions[@]}"; do
         local plugin_shim_name
         local plugin_shim_version