Added function to generate vkey from skey

[mhutchinson]

This is scoped only to Ed25519 keys. This comes up in many implementations where an operator wants to sign new messages, but also authenticate previous messages they may have made (e.g. witnesses, logs).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.14286% with 22 lines in your changes missing coverage. Please review.

Project coverage is 72.18%. Comparing base (fa00c16) to head (bc7d6d9).
Report is 113 commits behind head on main.

Files with missing lines	Patch %	Lines
note/note_verifier.go	37.14%	15 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #199       +/-   ##
===========================================
- Coverage   82.57%   72.18%   -10.39%     
===========================================
  Files           5        6        +1     
  Lines         241      471      +230     
===========================================
+ Hits          199      340      +141     
- Misses         30       93       +63     
- Partials       12       38       +26     

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[mhutchinson]

ta