Allow overwriting federated login session duration

**Is your feature request related to a problem? Please describe.**
Google SAML sets a session duration of 24h by default. I would like to reduce session duration to 8h.
Authentik prefers that session duration over one configured in the login stage.

**Describe the solution you'd like**
Allow overwriting the session duration in the configuration of SAML federated logins.

**Describe alternatives you've considered**
None

**Additional context**
None