Expose persistence package

[AlCutter]

This PR exposes the persistence implementations so that they can be used if folks want to build or embed their own witnesses.

Also moves the GCP Spanner persistence implementation over to sit alongside the others, and renames the sql implementation to sqlite to better reflect the dependency.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 24.12%. Comparing base (9ccbfe3) to head (fbb072c).

Files with missing lines	Patch %	Lines
persistence/inmemory/inmemory.go	62.50%	3 Missing ⚠️
cmd/omniwitness/monolith.go	0.00%	2 Missing ⚠️
cmd/omniwitness_gcp/main.go	0.00%	2 Missing ⚠️
persistence/spanner/persistence.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
- Coverage   24.37%   24.12%   -0.25%     
==========================================
  Files          30       30              
  Lines        2064     2064              
==========================================
- Hits          503      498       -5     
- Misses       1464     1469       +5     
  Partials       97       97              

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