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

[brustolin]

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

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[github-actions]

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## Unreleased

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

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Generated by 🚫 dangerJS against 279244d

[github-actions]

Performance metrics 🚀

	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
a176fc4	1251.52 ms	1259.14 ms	7.62 ms
df2986b	1227.45 ms	1244.35 ms	16.90 ms
257c2a9	1239.52 ms	1251.08 ms	11.56 ms
9fa25c2	1211.88 ms	1228.36 ms	16.48 ms
23e2db5	1240.96 ms	1253.86 ms	12.90 ms
533859f	1237.78 ms	1249.76 ms	11.98 ms
6943de0	1215.83 ms	1253.22 ms	37.39 ms
fd6a31c	1190.43 ms	1218.78 ms	28.35 ms
98cca71	1227.43 ms	1239.22 ms	11.79 ms
62c15d4	1231.80 ms	1248.86 ms	17.06 ms

App size

Revision	Plain	With Sentry	Diff
a176fc4	22.84 KiB	403.24 KiB	380.39 KiB
df2986b	22.85 KiB	406.89 KiB	384.04 KiB
257c2a9	20.76 KiB	401.36 KiB	380.60 KiB
9fa25c2	22.85 KiB	407.44 KiB	384.59 KiB
23e2db5	20.76 KiB	434.97 KiB	414.21 KiB
533859f	22.85 KiB	408.84 KiB	386.00 KiB
6943de0	20.76 KiB	393.33 KiB	372.57 KiB
fd6a31c	20.76 KiB	436.50 KiB	415.74 KiB
98cca71	22.85 KiB	411.14 KiB	388.29 KiB
62c15d4	22.85 KiB	411.14 KiB	388.29 KiB