Auto start app launch

[noahsmartin]

The SDK drops app start times if the app doesn't create a ui.load transaction within 5 seconds of app start. This causes a lot of user confusion and makes it difficult to get app launch measurements. This PR instead makes the SDK automatically send app start spans, so no extra code from the user is required to get it working.

One thing missing from the implementation is to suppress sending the auto created transaction if the span was already included in another transaction

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.480%. Comparing base (f0d57ae) to head (8dac8f3).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #6828       +/-   ##
=============================================
- Coverage   84.853%   84.480%   -0.373%     
=============================================
  Files          452       447        -5     
  Lines        27603     27301      -302     
  Branches     12104     11962      -142     
=============================================
- Hits         23422     23064      -358     
- Misses        4135      4189       +54     
- Partials        46        48        +2     

Files with missing lines	Coverage Δ
Sources/Sentry/SentryAppStartTracker.m	94.656% <100.000%> (+0.538%)	⬆️

... and 71 files with indirect coverage changes

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