Support post and pre hooks

[ccharlesgb]

Context

The dry runner does not evaluate whether post and pre hooks are valid syntax.
Adding this should be fairly easy, we just need to load the pre and post hook in the manifest and check if dry running the post/pre-hook will succeed.

Caveat

If the post/pre-hook runs DDL that creates/alters/deletes tables/views (i.e causes a schema change). Then the predicted schema of the dbt project will be incorrect. This could cause false positives/negatives in the dry run result. But this is not great practice as it is working around the materialisations provided by dbt.