Add and use richer interfaces for signer/subtreesigner.

[AlCutter]

This allows custom out-of-tree implementations, e.g. signers backend by HSMs, to be used across the transparency-dev repos, where formats/note Signers are used.

This is a breaking change, but likely to pass un-noticed in most uses of these signers/verifiers, but locations which create new signers and returned pointers to these type will need a small tweak to fix.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.58%. Comparing base (fa00c16) to head (34d6663).
⚠️ Report is 175 commits behind head on main.

Files with missing lines	Patch %	Lines
note/note_cosigv1.go	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #264      +/-   ##
==========================================
- Coverage   82.57%   76.58%   -6.00%     
==========================================
  Files           5        8       +3     
  Lines         241      696     +455     
==========================================
+ Hits          199      533     +334     
- Misses         30       90      +60     
- Partials       12       73      +61     

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[roger2hk]

Add new unit tests to cover the newly added NewMLDSASigner, NewMLDSAVerifier, or the algMLDSA44 case in NewSignerForCosignatureV1.

[roger2hk]

Suggested change

	type subtreeSigner struct {
	// subtreeSigner is a concrete implementation of the extended SubtreeSigner interface above.
	type subtreeSigner struct {

[AlCutter]

Add new unit tests to cover the newly added NewMLDSASigner, NewMLDSAVerifier, or the algMLDSA44 case in NewSignerForCosignatureV1.

Maybe I'm not understanding, but this fairly unrelated to the PR, which is just abstracting an interface... Those c'tors were already present, and there are also already tests for NewMLDSA and NewMLDSAVerifier:

$ grep -e "NewMLDSA\(Signer\|Verifier\)" note/*test.go
note/note_cosigv1_test.go:      signer, err := NewMLDSASigner(skey)
note/note_cosigv1_test.go:      verifier, err := NewMLDSAVerifier(vkey)
note/note_cosigv1_test.go:      signer, err := NewMLDSASigner(skey)
note/note_cosigv1_test.go:                      s, err := NewMLDSASigner(skey)
note/note_cosigv1_test.go:                              t.Fatalf("NewMLDSASigner(%q): %v", skey, err)
note/note_cosigv1_test.go:                      v, err := NewMLDSAVerifier(vkey)
note/note_cosigv1_test.go:                              t.Fatalf("NewMLDSAVerifier(%q): %v", vkey, err)

Happy to double back and add more tests later if you think they're necessary, though.