feat: support KERIpy-style /delegate/request EXN sending

[kentbull]

We need KERIpy-style /delegate/request EXN sending like line 199 in the Anchorer.partialWitnessEscrow in the main branch. This has been a missing feature in KERIA for a long time.

This delegation request EXN is sent from the delegate to the delegator in order for the delegator to get a notification that it can surface to a Signify UX where it shows the edge controller that a delegation request has come through.

In order for single sig delegation to have a KERIpy-style workflow this change is necessary.

I also added protocol, module, class, and function docs to clarify this change.

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.73%. Comparing base (2887167) to head (3d000ff).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/keria/app/delegating.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
+ Coverage   87.71%   87.73%   +0.02%     
==========================================
  Files          26       26              
  Lines        5810     5828      +18     
==========================================
+ Hits         5096     5113      +17     
- Misses        714      715       +1     

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