feat(session): Add unhandled session status type

[ryan953]

This is in service of this project: https://www.notion.so/sentry/RFC-Session-Status-unhandled-2308b10e4b5d801d9805fece61533ac5#2308b10e4b5d80e9b312ecbea27a4af3

TL/DR:
We're adding a new enum value to split up the existing crashed release health metric into two more specific metrics:

• Unhandled
• Crashed
  For cases like Flutter: a Dart error will not crash the running process, while a native error will. We want to be clearer about this when we report errors to sentry users.

This PR
This PR will allow us to properly count session and sessions envelopes that contain the new value. When a session envelope arrives that has the new value: status: unhandled we want the SessionLike metric to handle that the same way we handle status: abnormal or status: crashed. We'll subtract 1 from the errors count and return 1 from unhandled_count().

Similarly, for sessions envelopes we need to implement the unhandled_count() function.

Finally, we have to implement the if-statements that will get the new count from the session, session.unhandled_count() and push the value into the list.

Fixes REPLAY-549

[linear]

REPLAY-549 Update Relay to correctly count envelopes that contain the enum val