Migrate signers

[AlCutter]

This PR migrates the note signer/verifier implementations from transparency-dev/witness/internal/note with a small tweak to the API to use the embedded alg field in the pubkey.

[codecov]

Codecov Report

Attention: 42 lines in your changes are missing coverage. Please review.

Comparison is base (79a89ae) 100.00% compared to head (210e760) 81.73%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
note/note_cosigv1.go	74.07%	20 Missing and 8 partials ⚠️
note/note_verifier.go	79.10%	10 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              main      #66       +/-   ##
============================================
- Coverage   100.00%   81.73%   -18.27%     
============================================
  Files            3        5        +2     
  Lines           55      230      +175     
============================================
+ Hits            55      188      +133     
- Misses           0       30       +30     
- Partials         0       12       +12     

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[roger2hk]

LGTM assuming this is a direct copy from transparency-dev/witness/internal/note. Found some minor difference in note/note_verifier.go and note_verifier_test.go.

[AlCutter]

LGTM assuming this is a direct copy from transparency-dev/witness/internal/note. Found some minor difference in note/note_verifier.go and note_verifier_test.go.

I've changed the NewVerifier c'tor slightly as I don't think we need the extra param.

[AlCutter]

(@roger2hk sorry, will need another LTGM since I added another commit)