chore: Add way to force SDK Init in a background thread

[brustolin]

Tests for Unreal, I will write the description later if it works.

#skip_changelog

[github-actions[bot]]

	Fails
:no_entry_sign:	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

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## Unreleased

- Add way to force SDK Init in a background thread ([#3376](https://github.com/getsentry/sentry-cocoa/pull/3376))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by :no_entry_sign: dangerJS against 279244d03873bf7888c470fa0b28fdc2f958f85e

[github-actions[bot]]

Performance metrics :rocket:

	Plain	With Sentry	Diff
Startup time	1245.58 ms	1254.14 ms	8.56 ms
Size	22.85 KiB	411.93 KiB	389.08 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
a176fc448cf5f3ea7b188449b8170b14ce6805cf	1251.52 ms	1259.14 ms	7.62 ms
df2986b1b42cc83f32d8f9acb0139bf2f5f6c09f	1227.45 ms	1244.35 ms	16.90 ms
257c2a998df2f9a0e257802d69d4268d4a83c560	1239.52 ms	1251.08 ms	11.56 ms
9fa25c2f47d973ec4c113a10a8c81002d832a4be	1211.88 ms	1228.36 ms	16.48 ms
23e2db585acfb0529c726619e2d6e8acd020f487	1240.96 ms	1253.86 ms	12.90 ms
533859f7bae4a14f6c535a2ec7aef4f526f01e83	1237.78 ms	1249.76 ms	11.98 ms
6943de0faec429fe6e29ae8c9cdb31770ad05e94	1215.83 ms	1253.22 ms	37.39 ms
fd6a31ce3940224432d2ece19de8c052bec3c3d2	1190.43 ms	1218.78 ms	28.35 ms
98cca71ab965606c68f46b2f6a0a9c710ce6cdb2	1227.43 ms	1239.22 ms	11.79 ms
62c15d439cb3932ae1048f8a7b8ce93f6ed463fd	1231.80 ms	1248.86 ms	17.06 ms

App size

Revision	Plain	With Sentry	Diff
a176fc448cf5f3ea7b188449b8170b14ce6805cf	22.84 KiB	403.24 KiB	380.39 KiB
df2986b1b42cc83f32d8f9acb0139bf2f5f6c09f	22.85 KiB	406.89 KiB	384.04 KiB
257c2a998df2f9a0e257802d69d4268d4a83c560	20.76 KiB	401.36 KiB	380.60 KiB
9fa25c2f47d973ec4c113a10a8c81002d832a4be	22.85 KiB	407.44 KiB	384.59 KiB
23e2db585acfb0529c726619e2d6e8acd020f487	20.76 KiB	434.97 KiB	414.21 KiB
533859f7bae4a14f6c535a2ec7aef4f526f01e83	22.85 KiB	408.84 KiB	386.00 KiB
6943de0faec429fe6e29ae8c9cdb31770ad05e94	20.76 KiB	393.33 KiB	372.57 KiB
fd6a31ce3940224432d2ece19de8c052bec3c3d2	20.76 KiB	436.50 KiB	415.74 KiB
98cca71ab965606c68f46b2f6a0a9c710ce6cdb2	22.85 KiB	411.14 KiB	388.29 KiB
62c15d439cb3932ae1048f8a7b8ce93f6ed463fd	22.85 KiB	411.14 KiB	388.29 KiB

[philipphofmann]

@brustolin, I'm closing this due to inactivity. Feel free to reopen when you still need it.