feat: scene-load transctions based on SceneManagerAPI

[vaind]

See #235

This PR uses SceneManagerAPI which is only available in Unity 2020+

Samples:https://sentry.io/organizations/sentry-sdks/performance/sentry-unity:4e44bacb819a4509ac4bb20b2c53d154/?project=5439417&query=transaction.duration%3A%3C15m&statsPeriod=30d&transaction=unity.runtime.startup&unselectedSeries=p100%28%29

• move to SentryInitialization.cs (or use reflection?)
• figure out if we can add the integration before the first scene starts loading

The goal is to create a Transaction and a Span for loading scenes. These will be further extended by future auto instrumentation.

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[bitsandfoxes]

A remnant from having the smoke test as part of the sample project.

[vaind]

Maybe drop the "unity." prefix? Also, I've noticed the spans are missing names - what's the best practice in other SDKs?

https://sentry.io/organizations/sentry-sdks/performance/sentry-unity:4e44bacb819a4509ac4bb20b2c53d154/?project=5439417&query=transaction.duration%3A%3C15m&statsPeriod=30d&transaction=unity.runtime.startup&unselectedSeries=p100%28%29

[bitsandfoxes]

Maybe drop the "unity." prefix? Also, I've noticed the spans are missing names - what's the best practice in other SDKs?

https://sentry.io/organizations/sentry-sdks/performance/sentry-unity:4e44bacb819a4509ac4bb20b2c53d154/?project=5439417&query=transaction.duration%3A%3C15m&statsPeriod=30d&transaction=unity.runtime.startup&unselectedSeries=p100%28%29

Thanks for pointing that out.

Initially, I was a bit confused about the naming/description/operation of transactions and spans but the team came up with a scheme I think we can go with for now.