ref: Use forward declarations in header

[noahsmartin]

Cleans up a few imports which will make the swift migration a bit easier and improve the output of the migration script

#skip-changelog

Closes #6297

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.776%. Comparing base (119ab1c) to head (b079877).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #6296       +/-   ##
=============================================
+ Coverage   86.651%   86.776%   +0.124%     
=============================================
  Files          437       438        +1     
  Lines        37189     37320      +131     
  Branches     17345     17439       +94     
=============================================
+ Hits         32225     32385      +160     
+ Misses        4918      4889       -29     
  Partials        46        46               

Files with missing lines	Coverage Δ
...entry/Profiling/SentryProfiledTracerConcurrency.mm	91.439% <ø> (ø)
Sources/Sentry/SentryHub.m	96.701% <ø> (ø)
Sources/Sentry/SentryPerformanceTracker.m	100.000% <ø> (ø)
Sources/Sentry/SentryTracer.m	97.704% <ø> (+0.491%)	⬆️
...urces/Sentry/SentryUIEventTrackerTransactionMode.m	100.000% <ø> (ø)

... and 20 files with indirect coverage changes

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1225.45 ms	1251.96 ms	26.51 ms
Size	23.75 KiB	993.71 KiB	969.96 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
f43a482	1219.00 ms	1245.56 ms	26.56 ms
4be5cd8	1222.35 ms	1251.43 ms	29.07 ms
3279d4e	1215.76 ms	1256.45 ms	40.69 ms
4d264fa	1223.48 ms	1246.91 ms	23.44 ms
db9572a	1223.13 ms	1241.60 ms	18.47 ms
7d23639	1237.93 ms	1243.04 ms	5.11 ms
f2bfecd	1234.92 ms	1250.34 ms	15.42 ms
d3f650a	1225.45 ms	1241.86 ms	16.41 ms
7f2f69c	1237.61 ms	1266.96 ms	29.35 ms
1b9991e	1233.45 ms	1256.61 ms	23.17 ms

App size

Revision	Plain	With Sentry	Diff
f43a482	23.75 KiB	928.16 KiB	904.41 KiB
4be5cd8	23.75 KiB	980.80 KiB	957.06 KiB
3279d4e	23.75 KiB	938.32 KiB	914.57 KiB
4d264fa	23.74 KiB	874.07 KiB	850.33 KiB
db9572a	23.75 KiB	858.64 KiB	834.89 KiB
7d23639	23.75 KiB	891.01 KiB	867.26 KiB
f2bfecd	23.75 KiB	919.68 KiB	895.93 KiB
d3f650a	23.75 KiB	902.48 KiB	878.73 KiB
7f2f69c	23.75 KiB	913.38 KiB	889.63 KiB
1b9991e	23.75 KiB	908.01 KiB	884.26 KiB