feat: clean up gnokey add + add coverage (gnolang#1212)

## Description

This PR initially started out as adding support for account derivation
to `gnokey add`. However, over the discussions, it turned into a bigger
cleanup of the `gnokey add` subcommand suite.

Changes done:
- extracted ledger support into `gnokey add ledger`
- extracted multisig support into `gnokey add multisig`
- extracted bech32 public key support into `gnokey add bech32`
- added coverage (unit tests) for all functionality, even though they
were previously missing
- added the `--derive-path` flag to the base `gnokey add` (original
functionality of the PR)

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

.github/workflows/tm2.yml

@@ -79,7 +79,7 @@ jobs:
         working-directory: tm2
         run: |
           export GOPATH=$HOME/go
-          export GOTEST_FLAGS="-v -p 1 -timeout=20m -coverprofile=coverage.out -covermode=atomic"
+          export GOTEST_FLAGS="-v -p 1 -timeout=20m -coverprofile=coverage.out -covermode=atomic -tags='ledger_suite'"
           make ${{ matrix.args }}
           touch coverage.out
       - uses: actions/upload-artifact@v4

tm2/Makefile

@@ -20,7 +20,7 @@ GOFMT_FLAGS ?= -w
 # flags for `make imports`.
 GOIMPORTS_FLAGS ?= $(GOFMT_FLAGS)
 # test suite flags.
-GOTEST_FLAGS ?= -v -p 1 -timeout=30m
+GOTEST_FLAGS ?= -v -p 1 -timeout=30m -tags='ledger_suite'
 
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 # Dev tools