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 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.74%. Comparing base (93f6fb2) to head (a28b5e6).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/keria/app/delegating.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #432   +/-   ##
=======================================
  Coverage   87.74%   87.74%           
=======================================
  Files          26       26           
  Lines        5816     5825    +9     
=======================================
+ Hits         5103     5111    +8     
- Misses        713      714    +1     

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