feat: SSE broadcaster for agent events

[kentbull]

This adds AgentSignals, SseBroadcaster, SseBroadcasterDoer, SignalsStreamEnd , and SseEventIterable to support Signify clients connecting to a server sent events stream from a given Agent to any connected clients for that agent.

This is in preparation for future event driven features including the did:webs integration, among others. The SSE event stream is also a convenient way for clients to switch from long-polling to observing events sent from an agent. Long-polling can still be supported as a fallback, though SSE can be the default. This will significantly help with both bandwidth and performance on highly used Agency servers.

The SseEventIterable uses the same pattern as KERIpy's QryRpyMailboxIterable to leverage Falcon's rep.stream to open an SSE event stream to the client. Wrapping the SseBroadcaster.subscribe around the SseEventIterable allows HIO to asynchronously service the WSGI's synchronous SSE connection client.

[codecov]

Codecov Report

❌ Patch coverage is 96.70330% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.84%. Comparing base (2887167) to head (9810ff8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/keria/app/streaming.py	96.51%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
+ Coverage   87.71%   87.84%   +0.13%     
==========================================
  Files          26       27       +1     
  Lines        5810     5900      +90     
==========================================
+ Hits         5096     5183      +87     
- Misses        714      717       +3     

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[kentbull]

See WebOfTrust/signifypy#145 for the SignifyPy client implementation of this.

[kentbull]

And see the SignifyTS PR WebOfTrust/signify-ts#376 for the TypeScript client-side implementation of this.