feat: Runtime Init instrumentation

[bitsandfoxes]

Span & Transaction names need some discussion.

The goal here is to capture runtime initialization via spans i.e.

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## Unreleased

- Runtime Init instrumentation ([#991](https://github.com/getsentry/sentry-unity/pull/991))

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[vaind]

Just from a quick look at the picture first, so that I don't forget: maybe drop the "unity" prefix? Seems unnecessary

[vaind]

LGTM.

What's wrong with WebGL? Maybe add a comment to the class definition why it doesn't work there?

[bitsandfoxes]

LGTM.

What's wrong with WebGL? Maybe add a comment to the class definition why it doesn't work there?

Tracking the WebGL limitation here #1000
It seems to be an issue with the UnityWebRequestTransport that errors when trying to send events when the SDK is initialized during SubsystemRegistration.