feat: delete/remove credential endpoint

[iFergal]

Resolves #79. DELETE /credentials/{said} - it will remove the credential from the DB and any associated indices.

This is different to revoke, which is under DELETE /identifiers/{name}/credentials/{said} (the path used for issuance).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.78%. Comparing base (18d3ad7) to head (7b18cba).
Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #330      +/-   ##
==========================================
+ Coverage   93.06%   93.78%   +0.72%     
==========================================
  Files          36       36              
  Lines        7121     8030     +909     
==========================================
+ Hits         6627     7531     +904     
- Misses        494      499       +5     

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[2byrds]

LGTM