Adding test vector generation example

[kevinlewi]

Adding a new example (under examples/) for producing the hex-encoded byte representations of lookup and key history proofs.

This will allow other implementations to compare against and exercise protobuf deserialization / parsing in the event that there is another client-side implementation which can consume lookup and history proofs for verification.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.88%. Comparing base (3ce5335) to head (64bc708).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #445      +/-   ##
==========================================
- Coverage   88.61%   87.88%   -0.73%     
==========================================
  Files          39       38       -1     
  Lines        9109     8165     -944     
==========================================
- Hits         8072     7176     -896     
+ Misses       1037      989      -48     

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