Feat: Add client reports

[adinauer]

📜 Description

Adds recording discarded events and sending them with client reports to
Sentry. Discarded events give insight into where the SDK drops certain
events like, for example, beforeSend or rate limiting.

💡 Motivation and Context

Fixes #1894

💚 How did you test it?

Unit tests, Emulator, Some local load testing to ensure counts are correct when hit from multiple threads

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[codecov-commenter]

Codecov Report

Merging #1982 (2403eeb) into 6.x.x (be0ebe0) will increase coverage by 0.07%.
The diff coverage is 75.12%.

@@             Coverage Diff              @@
##              6.x.x    #1982      +/-   ##
============================================
+ Coverage     80.73%   80.81%   +0.07%     
- Complexity     3051     3125      +74     
============================================
  Files           220      228       +8     
  Lines         11266    11597     +331     
  Branches       1506     1551      +45     
============================================
+ Hits           9096     9372     +276     
- Misses         1603     1646      +43     
- Partials        567      579      +12     

Impacted Files	Coverage Δ
sentry/src/main/java/io/sentry/SentryOptions.java	81.17% <33.33%> (-1.25%)	⬇️
.../sentry/clientreport/NoOpClientReportRecorder.java	60.00% <60.00%> (ø)
...main/java/io/sentry/clientreport/ClientReport.java	62.00% <62.00%> (ø)
...in/java/io/sentry/clientreport/DiscardedEvent.java	63.15% <63.15%> (ø)
sentry/src/main/java/io/sentry/SentryItemType.java	86.66% <66.66%> (+1.48%)	⬆️
...n/java/io/sentry/clientreport/ClientReportKey.java	66.66% <66.66%> (ø)
...ry/transport/apache/ApacheHttpClientTransport.java	71.87% <68.85%> (+2.54%)	⬆️
...a/io/sentry/clientreport/ClientReportRecorder.java	74.32% <74.32%> (ø)
...ry/src/main/java/io/sentry/SentryEnvelopeItem.java	86.71% <81.25%> (-0.69%)	⬇️
...src/main/java/io/sentry/transport/RateLimiter.java	85.85% <83.33%> (+9.22%)	⬆️
... and 18 more

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[philipphofmann]

This is a high-level review of the code. I didn't look at any tests or too close to the code. Overall this PR looks great. Thanks for doing this @adinauer 👏

[philipphofmann]

Nothing to add.

[marandaneto]

If DataCategory is used outside of the transport layer, we can move out to the root sentry folder.

It's ok having a clientreport package and public classes as long as they are marked as internal with annotations.

Classes that are not used outside of the client report can be package-private.

Public methods that are in public classes and need to be used in different packages, can also be marked as internal with annotations, e.g. Options#getClientReportRecorder.

@bruno-garcia opinions?

[philipphofmann]

Didn't see any issues from a high-level perspective. Thanks for doing this 💪