Add user feedback capability to the Native SDK

[tustanivsky]

This PR introduces a new API allowing to create/capture user feedback for specific events (similar functionality is offered by native Android and Apple SDKs).

It will be later exposed to the Unreal Engine plugin where user feedback capabilities have been requested recently. While the existing user feedback endpoint provides an alternative it requires deviating from our current approach of utilizing the native SDKs for client app communication with Sentry there.

Related to #885

[codecov]

Codecov Report

Attention: Patch coverage is 80.88235% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 82.63%. Comparing base (e812e7c) to head (1b6c03c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #966      +/-   ##
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- Coverage   82.65%   82.63%   -0.03%     
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  Files          53       53              
  Lines        7376     7444      +68     
  Branches     1186     1198      +12     
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+ Hits         6097     6151      +54     
- Misses       1171     1182      +11     
- Partials      108      111       +3     

[supervacuus]

This looks great! Thanks a lot! One minor change.

I think this should also find its way into the docs.

[tustanivsky]

@supervacuus Thanks for the prompt feedback! I've updated the integration tests as you suggested and opened a corresponding PR for the docs.