Add dynamic ctlog verifier selection based on URL and correct STH log_id generation

[fghanmi]

Summary

This update removes the previously hardcoded CTlog key ID and instead dynamically selects the correct CTlog verifier based on the CTlog's BaseURL. It also fixes the issue where STHs generated by the monitor contained an empty log_id field by correctly computing the log ID as the SHA-256 hash of the DER-encoded public key. These changes allow ct_monitor to work properly with both custom CT logs and the public Sigstore CT log instances.

This PR also includes a variable rename, fulcioClient -> ctlogClient

Issue: #829

Release Note

Documentation

[codecov]

Codecov Report

❌ Patch coverage is 16.12903% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.75%. Comparing base (d271ec7) to head (85f45de).
⚠️ Report is 428 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/ct/consistency.go	18.51%	20 Missing and 2 partials ⚠️
cmd/ct_monitor/main.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #830       +/-   ##
===========================================
- Coverage   64.02%   35.75%   -28.28%     
===========================================
  Files           4       24       +20     
  Lines         303     2302     +1999     
===========================================
+ Hits          194      823      +629     
- Misses         78     1382     +1304     
- Partials       31       97       +66     

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