Feat: Add support for pre, post, on_virtual_update statements in config

[themisvaltinos]

This update enables defining pre_statements, post_statements and on_virtual_update statements at the project level in the configuration that will be applied to all models.

Example:

    model_defaults:
      dialect: duckdb
      pre_statements:
        - "SET timeout = 300000"
      post_statements:
        - "@IF(@runtime_stage = 'evaluating', ANALYZE @this_model)"
      on_virtual_update:
        - "GRANT SELECT ON @this_model TO ROLE analyst_role"

These default statements are merged with any model-specific statements, with default statements executing first followed by the model-specific ones.

[erindru]

Looks good to me!

[georgesittas]

Suggested change

	assert model.python_env["__sqlmesh__vars__"].payload == "{'var1': 4}"
	assert model.python_env[c.SQLMESH_VARS].payload == "{'var1': 4}"