AddEventListener support for IDesktopAgent and IPrivateChannel

[bingenito]

Implementation of core event system of FDC3 2.2

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[bingenito]

I will add tests for the concrete event classes to ensure constructor initialization is done correctly

[kriswest]

LGTM - but needs the UserChannelChanged event to fire on leaving the channel as well as switching channel

[kemerava]

@bingenito Thanks for working on this! So just to clarify, are you proposing to just stick with the Fdc3Event and not create the type of the IApiEvent? I see how it does not add much value, especially if you use your definitions of IFdc3Event and Fdc3Event so I am fine with modifying it that way
Please let me know if I am correct in my understading and I can modify my PR

[psmulovics]

@kemerava I am with @bingenito on this - C# (and I think Java too) do not have the expressiveness to have the same notion as is expressed in the TS definition when comes to the typedness of these

[kriswest]

One unimplemented function (removing listeners), a question and a minor typo.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.95%. Comparing base (efb171e) to head (4228190).
Report is 131 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Fdc3/Fdc3Event.cs	95.45%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #170      +/-   ##
==========================================
+ Coverage   75.93%   77.95%   +2.02%     
==========================================
  Files          55       63       +8     
  Lines         453      549      +96     
  Branches       49       49              
==========================================
+ Hits          344      428      +84     
- Misses         87       98      +11     
- Partials       22       23       +1     

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[kriswest]

Associated FDC3 docs PR:

• Adding .NET docs for Events to API reference FDC3#1441

[kriswest]

LGTM