fix: collect main thread data synchronously during init

[vaind]

After adding the exception handling to SentryInitialization.cs, I've noticed it doesn't collect any of the usual context. This fixes that (and maybe a few other very early user-space error captures). The original behaviour was to run the main thread info collection asynchronously in a coroutine. Now, we collect it right away during init.

Before

https://sentry.io/organizations/sentry-sdks/issues/3267829133/events/2651b13a859b4859860b3f44859ffacd/?project=5439417

After

https://sentry.io/organizations/sentry-sdks/issues/3267829133/events/e12e6634035b4eb0bdc3c4349a5d03e1/?project=5439417

[bitsandfoxes]

The original reason for the callback solution: #233 (comment)
Can we measure the actual impact once?
@bruno-garcia any thoughts?

[vaind]

Can we measure the actual impact once?

I've done a rough measurement with the stopwatch (haven't done this in c# before so didn't look up if there's a better tool). In the editor, the runtime of CollectData(), after removing the yields, ~0.002 sec, i.e. 2 milliseconds, but that's an unoptimized build. In a desktop build, it was immeasurable (0.000) without adding an artificial loop. I could run it on mobile, though from the actual APIs we use, I doubt there would be much of a difference.