Add a websocket API to the integration for event retain

[dermotduffy]

• As required by Allow snapshots/clips to be starred from the card dermotduffy/frigate-hass-card#743

[codecov]

Codecov Report

Merging #295 (8bacf6a) into master (ef7f549) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #295   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        13    +1     
  Lines         1489      1518   +29     
=========================================
+ Hits          1489      1518   +29     

Impacted Files	Coverage Δ
custom_components/frigate/__init__.py	100.00% <100.00%> (ø)
custom_components/frigate/api.py	100.00% <100.00%> (ø)
custom_components/frigate/media_source.py	100.00% <100.00%> (ø)
custom_components/frigate/views.py	100.00% <100.00%> (ø)
custom_components/frigate/ws_api.py	100.00% <100.00%> (ø)

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[dermotduffy]

Thanks for the review @NickM-27 !