Feat: Check freshness for mixed external & sqlmesh models

[VaggelisD]

Follow up PR on model freshness (Previous PR)

This PR enables mixed SQLMesh & external (source) model freshness, e.g:

A: External model 
B: SELECT * FROM A
C: SELECT * FROM A, B

Model C will be evaluated if its upstream models are fresh, which is determined by:

• C's last_altered_ts compared to external model's (A) INFORMATION_SCHEMA metadata
• The existence (or not) of parent intervals for SQLMesh models (B), signalling if any model will be evaluated as part of this run

A new freshness-specific test file is added to break up the original test in various scenarios.