feat: add generic agent SSE signaling client

[kentbull]

This adds the SignifyTS client counterpart to KERIA's SSE AgentSignals, broadcaster, SSE endpoint, and SSE iterable. This supports one or more Signify clients connecting to a given server Agent to get unique server sent events streams per SignifyClient controller instance.

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 is still be supported as a fallback, though this allows SSE to be the default future communication pathway from the agent to a client. This will significantly help with both bandwidth and performance on highly used Agency servers.

This PR should be merged along side WebOfTrust/keria #435 WebOfTrust/keria#435

[kentbull]

WebOfTrust/signifypy#145 is the SignifyPy correlate to this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.53%. Comparing base (90a8701) to head (c723fc3).

Additional details and impacted files

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  Coverage   97.53%   97.53%           
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  Files           1        1           
  Lines         325      325           
  Branches       24       24           
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  Hits          317      317           
  Misses          8        8           

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