feat: add support for eventing

[bacherfl]

This PR adds support for Eventing and thus solves a part of #126

Note: I had to add some Thread.Sleeps in the unit tests to verify the event handlers, which are called by a separate thread, are called eventually. Coming from a Golang background, I would have liked to have something like require.Eventually, but I did not find anything similar for dotnet. If anyone with more dotnet experience knows how to do this in a better way, I appreciate any suggestions :)

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (24c3441) 94.30% compared to head (2494599) 94.73%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
src/OpenFeature/EventExecutor.cs	95.29%	6 Missing and 6 partials ⚠️
src/OpenFeature/Api.cs	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   94.30%   94.73%   +0.42%     
==========================================
  Files          21       23       +2     
  Lines         650      931     +281     
  Branches       54       93      +39     
==========================================
+ Hits          613      882     +269     
- Misses         23       29       +6     
- Partials       14       20       +6     

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[toddbaert]

Really great job! Excellent testing and very readable overall. Left a few comments. I think my only blocker is the test I described here.

[toddbaert]

Thanks @bacherfl ! Recent changes have me satisfied.

[Kavindu-Dodan]

Approved the PR. Feel free to resolve my comments and merge. Nice work @bacherfl 🎉

[toddbaert]

I'll merge this EOD Tuesday next week unless I hear objections!

I think we can then consider our pending 2.0 release.