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 :white_check_mark:

Project coverage is 87.88%. Comparing base (3ce5335) to head (64bc708). Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ 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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