chore(profiling): place legacy compilation gates around profiling code that will need to be changed/removed

[armcknight]

To kick off work for #3555, introduce two new preprocessor definitions to help transition the profiling implementation from the current form (henceforth known as "legacy") to continuous mode.

Gate declarations and implementation that will change or be removed with the legacy flag. This is not necessarily exhaustive, but serves as a guidepost for further work.

#skip-changelog

[codecov]

Codecov Report

Attention: Patch coverage is 86.36364% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 89.541%. Comparing base (b15521e) to head (04e8a2f).

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3808       +/-   ##
=============================================
- Coverage   89.588%   89.541%   -0.048%     
=============================================
  Files          560       560               
  Lines        60819     60809       -10     
  Branches     21876     21862       -14     
=============================================
- Hits         54487     54449       -38     
- Misses        5296      5318       +22     
- Partials      1036      1042        +6     

Files	Coverage Δ
Sources/Sentry/PrivateSentrySDKOnly.mm	87.155% <ø> (ø)
...entry/Profiling/SentryProfiledTracerConcurrency.mm	94.736% <100.000%> (ø)
Sources/Sentry/SentryHub.m	98.613% <100.000%> (ø)
Sources/Sentry/SentryProfileTimeseries.mm	48.076% <ø> (ø)
Sources/Sentry/SentrySampling.m	93.333% <ø> (ø)
Sources/Sentry/SentryTimeToDisplayTracker.m	100.000% <100.000%> (ø)
Sources/Sentry/SentryTracer.m	96.226% <100.000%> (-0.172%)	⬇️
Sources/Sentry/SentryTransactionContext.mm	100.000% <100.000%> (ø)
...entryProfilerTests/SentryAppLaunchProfilingTests.m	100.000% <100.000%> (ø)
Tests/SentryProfilerTests/SentryBacktraceTests.mm	94.904% <ø> (+0.099%)	⬆️
... and 8 more

... and 12 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b15521e...04e8a2f. Read the comment docs.

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1236.24 ms	1250.27 ms	14.02 ms
Size	21.58 KiB	573.18 KiB	551.60 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
c319795	1256.18 ms	1266.87 ms	10.69 ms
de033da	1206.55 ms	1217.08 ms	10.53 ms
97797b4	1243.47 ms	1252.84 ms	9.37 ms
d3630c3	1248.52 ms	1262.70 ms	14.18 ms
075a044	1237.26 ms	1255.36 ms	18.10 ms
ef5821b	1253.18 ms	1265.46 ms	12.28 ms
b9a9ffd	1201.61 ms	1215.06 ms	13.45 ms
e2abb0d	1235.08 ms	1257.00 ms	21.92 ms
7512778	1200.78 ms	1206.33 ms	5.55 ms
4af7581	1204.67 ms	1241.33 ms	36.66 ms

App size

Revision	Plain	With Sentry	Diff
c319795	20.76 KiB	431.99 KiB	411.22 KiB
de033da	21.58 KiB	418.15 KiB	396.57 KiB
97797b4	22.85 KiB	408.72 KiB	385.88 KiB
d3630c3	22.84 KiB	403.19 KiB	380.34 KiB
075a044	20.76 KiB	420.41 KiB	399.65 KiB
ef5821b	21.58 KiB	414.96 KiB	393.37 KiB
b9a9ffd	21.58 KiB	418.43 KiB	396.85 KiB
e2abb0d	20.76 KiB	434.72 KiB	413.96 KiB
7512778	21.58 KiB	539.87 KiB	518.29 KiB
4af7581	22.84 KiB	401.62 KiB	378.78 KiB

[philipphofmann]

I don't want #3808 (review) to block this PR. If you would like to stick to compiler flags, which I don't think is a great idea, do so @armcknight.

[armcknight]

OK, we're going to go with the feature flag approach in #3831