Query endpoint and database seeker (indexer) for Exchange messages.

[pfeairheller]

This PR includes:

• New seeker database for exchange messages, indexing on sender, recipient, route, and date sent.
• Endpoint for filtering, sorting and paginating exn messages.

[codecov]

Codecov Report

Merging #107 (694bf58) into development (05c7eab) will increase coverage by 3.48%.
Report is 9 commits behind head on development.
The diff coverage is 93.25%.

@@               Coverage Diff               @@
##           development     #107      +/-   ##
===============================================
+ Coverage        88.20%   91.68%   +3.48%     
===============================================
  Files               30       35       +5     
  Lines             4789     5543     +754     
===============================================
+ Hits              4224     5082     +858     
+ Misses             565      461     -104     

Files	Coverage Δ
src/keria/app/aiding.py	85.03% <100.00%> (+7.29%)	⬆️
src/keria/app/notifying.py	100.00% <100.00%> (ø)
src/keria/app/presenting.py	96.61% <100.00%> (+1.69%)	⬆️
src/keria/testing/testing_helper.py	99.35% <100.00%> (+0.06%)	⬆️
tests/app/test_agenting.py	97.12% <100.00%> (+0.54%)	⬆️
tests/app/test_aiding.py	100.00% <100.00%> (ø)
tests/app/test_basing.py	94.73% <100.00%> (+4.17%)	⬆️
tests/app/test_credentialing.py	99.72% <100.00%> (+0.02%)	⬆️
tests/app/test_grouping.py	100.00% <100.00%> (ø)
tests/app/test_indirecting.py	100.00% <100.00%> (ø)
... and 11 more

... and 2 files with indirect coverage changes

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