feat: agent config retrieval

[iFergal]

Simple endpoint to retrieve the agent JSON config.

The motivation for this is a really low effort way to discover the witnesses etc that KERIA is aware of when deployed, as described in #301. There might be better solutions that this but it will be OK for my needs for now.

This allows a Signify client to discover from iurls witnesses, watchers etc, provided the OOBI URLs are given an appropriate tag as a query param.

I only exposed iurls but named it as a config endpoint so more can be exposed in the future if it makes sense.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.76%. Comparing base (18d3ad7) to head (9c5d59d).
Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #318      +/-   ##
==========================================
+ Coverage   93.06%   93.76%   +0.70%     
==========================================
  Files          36       36              
  Lines        7121     8001     +880     
==========================================
+ Hits         6627     7502     +875     
- Misses        494      499       +5     

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[2byrds]

Nice! LGTM.