Implement SignSubtree in witness client

[roger2hk]

This pull request implements the /sign-subtree endpoint in witness client according to the tlog-witness specification.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.45455% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.92%. Comparing base (61e4168) to head (e233842).

Files with missing lines	Patch %	Lines
client/http/witness_client.go	70.45%	8 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
+ Coverage   24.91%   25.92%   +1.01%     
==========================================
  Files          29       29              
  Lines        2408     2449      +41     
==========================================
+ Hits          600      635      +35     
- Misses       1680     1681       +1     
- Partials      128      133       +5     

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