fix: add platform to chunk envelope item headers

[armcknight]

The profiling team needs to be able to rate limit different platform SDKs differently, but can't simply use the SDK info name field in Relay, since some platforms have multiple different SDKs, like the JS ecosystem.

Related to getsentry/sentry-javascript#15758 and getsentry/sentry-java#4287

#skip-changelog

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.719%. Comparing base (fbb7656) to head (9b532a1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/Sentry/SentryEnvelopeItemHeader.m	66.666%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #5041       +/-   ##
=============================================
+ Coverage   92.711%   92.719%   +0.007%     
=============================================
  Files          672       672               
  Lines        82298     82304        +6     
  Branches     29981     29972        -9     
=============================================
+ Hits         76300     76312       +12     
+ Misses        5898      5894        -4     
+ Partials       100        98        -2     

Files with missing lines	Coverage Δ
...es/Sentry/Profiling/SentryProfilerSerialization.mm	85.823% <100.000%> (+0.054%)	⬆️
...yProfilerTests/SentryContinuousProfilerTests.swift	97.976% <100.000%> (+0.005%)	⬆️
Sources/Sentry/SentryEnvelopeItemHeader.m	97.222% <66.666%> (-2.778%)	⬇️

... and 9 files with indirect coverage changes

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1223.49 ms	1247.98 ms	24.49 ms
Size	22.30 KiB	832.42 KiB	810.11 KiB