fix: Native layer not closing

[bitsandfoxes]

On mobile, the native layer is getting set up during buildtime and is self-initializing.
If the SDK gets disabled during runtime (i.e. in the Configure callback, then the native layer needs to be closed down too.

Additionally, there was no way to close the SDK and have the native layer close with it.

[vaind]

It's OK, though maybe we could clean SentryUnity class a bit by extracting a non-static class with all the current code and have a single variable private static SentryUnitySdk? Instance that we set in init() and remove in close() (+ call its close() of course).
SentryUnitySdk could have non-static and non-nullable Options, SentrySdk and FileStream lockfile`
WDYT?

[bitsandfoxes]

It's OK, though maybe we could clean SentryUnity class a bit by extracting a non-static class with all the current code and have a single variable private static SentryUnitySdk? Instance that we set in init() and remove in close() (+ call its close() of course). SentryUnitySdk could have non-static and non-nullable Options, SentrySdk and FileStream lockfile` WDYT?

Excellent point.