Support other databases besides sqlite3 in SqlCache

[pcoccoli]

Is your feature request related to a problem? Please describe.
Since #529 is done, we should now make SqlCache database-agnostic. Currently it still only works with sqlite3, as it uses the session ID as a filename and creates a SQLAlchemy "connection string" for sqlite.

Describe the solution you'd like
We should be able to specific a cache config (location) in kestrel.yaml so that we can use other DBs, like PostgreSQL. That config should either be a filename (so we use that name as the sqlite database file), direcory (we put a {session_id}.db in that directory), or a SQLAlchemy connection string (which we just use as-is).

Describe alternatives you've considered
Can't think of anything else

Additional context