Enhance SQLMesh to Support Audits for External Sources

[harshvardhan-j]

Description:
Currently, SQLMesh supports external sources but lacks the functionality to perform audits on them. The schema.yaml file for external models does not include an interface for adding audits, limiting the ability to ensure data integrity and reliability for downstream tables.

Objective:
Implement a feature within SQLMesh to enable audits for external sources, allowing users to verify that downstream tables receive the expected data from these external models.

Context: https://tobiko-data.slack.com/archives/C044BRE5W4S/p1712156303563069?thread_ts=1712149507.981779&cid=C044BRE5W4S