fix: use exn.rp field as recipient for exchange message (#3)

[lenkan]

All exchange messages now have the intended recipient in the payload, so the extra body parameter is confusing and could cause issues.

This PR removes the "rec" body parameter that previously specified a list of recipients of an exn message. This list had no correlation with the rp field in the exchange message payload.

This PR also removes the ipex multisig handlers, for no other reason that they weren't used in any of the signify integration tests, so to me it was unnecessary complexity. I do not see any reason to keep this feature as we already use the "/multisig/exn" from the client.

See discussions:

• Fix/test del rot multisig more signify-ts#267 (comment)
• Update POST /identifiers/name/exchanges endpoint to report recp field in POST body #270
• Bug: Exchanges.send() should not return early and instead execute for each recipient signify-ts#310
• fix: send exn for each recipient; no early return signify-ts#311

[codecov]

Codecov Report

❌ Patch coverage is 67.64706% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.32%. Comparing base (1f22907) to head (eed1c22).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/keria/app/agenting.py	34.37%	21 Missing ⚠️
src/keria/peer/exchanging.py	75.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (67.64%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (84.32%) is below the target coverage (85.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
- Coverage   86.60%   84.32%   -2.29%     
==========================================
  Files          25       25              
  Lines        5286     5122     -164     
==========================================
- Hits         4578     4319     -259     
- Misses        708      803      +95     

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