Multisig join for KERIA agents

[pfeairheller]

This PR includes:

• Addition of a join endpoint that can be used by participants being asked to contribute keys to a rotation to join an existing group multisig.

[codecov]

Codecov Report

Merging #123 (39a007f) into development (f80d976) will decrease coverage by 0.11%.
Report is 1 commits behind head on development.
The diff coverage is 86.43%.

@@               Coverage Diff               @@
##           development     #123      +/-   ##
===============================================
- Coverage        91.77%   91.66%   -0.11%     
===============================================
  Files               37       37              
  Lines             5847     5987     +140     
===============================================
+ Hits              5366     5488     +122     
- Misses             481      499      +18     

Files	Coverage Δ
src/keria/app/credentialing.py	80.86% <100.00%> (ø)
tests/app/test_agenting.py	97.23% <100.00%> (+0.10%)	⬆️
tests/app/test_basing.py	94.94% <100.00%> (+0.21%)	⬆️
tests/app/test_credentialing.py	99.72% <100.00%> (-0.01%)	⬇️
tests/app/test_grouping.py	100.00% <100.00%> (ø)
tests/app/test_ipexing.py	100.00% <100.00%> (ø)
src/keria/app/ipexing.py	98.52% <88.88%> (+3.00%)	⬆️
src/keria/db/basing.py	94.08% <75.00%> (-0.17%)	⬇️
src/keria/app/grouping.py	83.08% <88.67%> (+3.32%)	⬆️
src/keria/app/agenting.py	80.96% <48.48%> (-1.58%)	⬇️

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