Add functionality for finding subjects in certificate transparency logs

[linus-sun]

Summary

This PR, per work outlined here, begins implementation of supporting identity monitoring within certificate transparency logs by adding support for monitoring subjects within a certificate in a certificate transparency log's entry. Functionality is tested via unit testing.

Release Note

NONE

Documentation

N/A

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 20 lines in your changes missing coverage. Please review.

Project coverage is 61.93%. Comparing base (d271ec7) to head (f3e7f7f).
Report is 150 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/ct/test_utils.go	28.57%	8 Missing and 2 partials ⚠️
pkg/ct/monitor.go	59.09%	8 Missing and 1 partial ⚠️
pkg/util/file/file.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #514      +/-   ##
==========================================
- Coverage   64.02%   61.93%   -2.09%     
==========================================
  Files           4       17      +13     
  Lines         303     1248     +945     
==========================================
+ Hits          194      773     +579     
- Misses         78      400     +322     
- Partials       31       75      +44     

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