Feat!: Skip model evaluation if upstream external model(s) have not changed

[VaggelisD]

This PR enables SQLMesh to skip re-evaluating models that depend on external model(s) which have not been updated since the last run.

This is done by:

1. Adding the last_altered_ts field in the _intervals table which tracks the last modification to the physical table of that snapshot
2. Querying the INFORMATION_SCHEMA for the external model's "last altered" timestamp wherever applicable (BQ & Snowflake for the time being)

Edit n.1: The freshness check was refactored into a built-in signal to (1) make the feature opt-in and (2) matches conceptually and the rest of the UX is given to us "for free".