Support custom dispatchers for event streams

[peterbroadhurst]

I have a use of this library, where I need to handle events within the Go code directly - using constructs specific to the Microservice.

Unlike the Webhook and WebSocket implementations, these cannot be made generic in the common library. So instead, I propose adding this extension point to the common library to allow me to extended it with my implementation.

This PR also proposes some quality of life improvements, particularly around name-or-UUID semantics, and upsert.

• Also proposes a small extra utility that supports generics for returning {items:[]} style responses
• Also fixes 64a7088 without which the eventsreams function breaks on restart when you have un-set batchTimeout, with:
  FF00234: Value '0' is invalid for field 'batchTimeout'

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fa739e9) 99.98% compared to head (128dec8) 99.98%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #117   +/-   ##
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  Coverage   99.98%   99.98%           
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  Files          78       78           
  Lines        6370     6416   +46     
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+ Hits         6369     6415   +46     
  Misses          1        1           

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