Adds an RFC6962-via-sunlight feeder

[AlCutter]

This PR adds support for witnessing RFC6962 logs by converting their Signed Tree Head structures into checkpoints using the sunlight RFC6962 STH signing algorithm.

This probably isn't super useful at the moment, but will become more-so with bastion support, and is a good PoC that the conversion works.

TODO: Needs transparency-dev/formats#99

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 62 lines in your changes missing coverage. Please review.

Project coverage is 36.69%. Comparing base (3c58af4) to head (483b318).
Report is 183 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/feeder/rfc6962/rfc6962_feeder.go	0.00%	60 Missing ⚠️
omniwitness/omniwitness.go	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (3c58af4) and HEAD (483b318). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (3c58af4)	HEAD (483b318)
	4	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #182       +/-   ##
===========================================
- Coverage   51.05%   36.69%   -14.36%     
===========================================
  Files          11       22       +11     
  Lines         903     1210      +307     
===========================================
- Hits          461      444       -17     
- Misses        374      675      +301     
- Partials       68       91       +23     

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