On run end

[mpcarter]

Add feature similar to dbt's on-run-end and on-run-start functionality.
https://docs.getdbt.com/reference/project-configs/on-run-start-on-run-end

I think ideally it would allow some way to run macros or macro-enabled sql at the start and/or end of sqlmesh plan or sqlmesh run.
The most common use case to run any set up or wrap up code that is not specific to a single model.

E.g. granting usage to schemas.
UDFs used in multiple models.