Strip name prefix from witness URLs when forwarding

[AlCutter]

This PR changes the LB config such that requests forwarded to individual witness instances are rewritten to match the expected path of /add-checkpoint with no prefix.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 22.63%. Comparing base (3c58af4) to head (ab2a967).
⚠️ Report is 292 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #492       +/-   ##
===========================================
- Coverage   51.05%   22.63%   -28.42%     
===========================================
  Files          11       28       +17     
  Lines         903     1975     +1072     
===========================================
- Hits          461      447       -14     
- Misses        374     1451     +1077     
- Partials       68       77        +9     

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