Add support for netrc in jvm_maven_import_external (#1509)

`auth=` parameter for `repository_ctx.download` is both
required to fetch from private registries and is not well-documented
bazelbuild/bazel#13709 (comment)

With this change
```python
rules_scala_toolchain_deps_repositories(
    maven_servers = ["some_private_artifactory_url"]
)
```
would be able to authenticate using default .netrc (like `~/.netrc`)

scala/scala_maven_import_external.bzl

@@ -35,6 +35,17 @@ the following macros are defined below that utilize jvm_import_external:
 - java_import_external - to demonstrate that the original functionality of `java_import_external` stayed intact.
 """
 
+load("@bazel_tools//tools/build_defs/repo:utils.bzl", "read_netrc", "read_user_netrc", "use_netrc")
+
+# https://github.com/bazelbuild/bazel/issues/13709#issuecomment-1336699672
+def _get_auth(ctx, urls):
+    """Given the list of URLs obtain the correct auth dict."""
+    if ctx.attr.netrc:
+        netrc = read_netrc(ctx, ctx.attr.netrc)
+    else:
+        netrc = read_user_netrc(ctx)
+    return use_netrc(netrc, urls, ctx.attr.auth_patterns)
+
 _HEADER = "# DO NOT EDIT: generated by jvm_import_external()"
 _PASS_PROPS = (
     "neverlink",
@@ -109,7 +120,7 @@ def _jvm_import_external(repository_ctx):
         lines.append(extra)
         if not extra.endswith("\n"):
             lines.append("")
-    repository_ctx.download(urls, path, sha)
+    repository_ctx.download(urls, path, sha, auth = _get_auth(repository_ctx, urls))
     if srcurls and _should_fetch_sources_in_current_env(repository_ctx):
         repository_ctx.download(srcurls, srcpath, srcsha)
     repository_ctx.file("BUILD", "\n".join(lines))
@@ -227,6 +238,8 @@ jvm_import_external = repository_rule(
             default = ["//visibility:public"],
         ),
         "extra_build_file_content": attr.string(),
+        "auth_patterns": attr.string_dict(),
+        "netrc": attr.string(),
     },
     environ = [_FETCH_SOURCES_ENV_VAR_NAME],
 )