feat: cleanup temp resources with session deletion

[shobsi]

BEGIN_COMMIT_OVERRIDE
feat: cleanup temp resources with session deletion (#1068)

chore: improve cleanup of test remote functions (#1068)
END_COMMIT_OVERRIDE

This change expands the current cleanup process which applies only to the default US region and only those test pipelines that run the tests depending on the fixtures. Namely, it:

1. Adds the Session class destructor, and calls close() from it.
2. Add a nox session for cloud function cleanup and run it with doctest. This cleans up in all regions in the project.

Fixes internal issue 371360614 🦕