feat: Cache Unhandled session instead of sending it immediately

[bitsandfoxes]

Resolves #4632
Followup on #4633

Problem

Sessions that end in Unhandled can later on still crash. Updating a session with an end status is a terminal operation, see getsentry/sentry-docs#15086 and once updated, sessions must not receive further updates. We need to make sure the SDK finishes a session only when it is confident, that a session is actually finished.

Proposal

Instead of updating (and ending) a session with SessionEndStatus.Unhandled right away we persist the state to disk. We can make use of the fact that there is a separation between PersistedSessionUpdate and SessionUpdate.

• PersistedSessionUpdate is used locally only, the SDK does not send this to Sentry and we can add a flag PendingUnhandled to it
• SessionUpdate is the actual update the SDK sends to Sentry

The flow is as follows:

1. Unhandled non-terminal exception captured (i.e. via Unity loghandling)
2. The client triggers the session to be marked as Unhandled and the update is persisted to disk
3. If the app crashes we end the session as Crashed, overwriting the pending unhandled state
4. If the app exits normally, we check for the endstatus and overwrite Exited with Unhandled.

TODO

Currently, the SDK relies on restarts of the application to send the SessionEndStatus. Ideally, we would also hook into some shutdown mechanism, or at the very least, closing the SDK should also end any active sessions.

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