Get attestation from ETS or Session now checks for expiry.

[idyll]

A Samly.Assertion cached in ETS or the Session will eventually expire.

Samly.State.Store.get_assertion/3 will return currently expired sessions. This is problematic because the Samly.AuthHandler looks for the cached session and doesn't replace it even if it is expired.

This is further exacerbated by Samly.get_active_assertion/1 returning expired assertions.

To address this I've updated the session and ETS logic to no longer return an expired session.
I contemplated purging ETS based on time, but it's most likely that the session store is being used in production.

[CLAassistant]

All committers have signed the CLA.

[k-cross]

reviewed, and thank you for the fix!