Expose core tlog-witness

[AlCutter]

This PR exposes the core witness and HTTP support, which together represent the tlog-witness implementation, in a new /witness package.

Some unused/obsolete code was removed during the process.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.44444% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 23.02%. Comparing base (e288f04) to head (02f3d17).

Files with missing lines	Patch %	Lines
witness/http.go	53.33%	7 Missing ⚠️
omniwitness/omniwitness.go	40.00%	5 Missing and 1 partial ⚠️
internal/persistence/testonly/persistence.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
- Coverage   24.20%   23.02%   -1.19%     
==========================================
  Files          31       30       -1     
  Lines        2115     2063      -52     
==========================================
- Hits          512      475      -37     
+ Misses       1515     1499      -16     
- Partials       88       89       +1     

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