feature: teach cargo pgrx schema how to emit just the SQL for named entities

[eeeebbbbrrrr]

I've found this feature super helpful in development of an extension.

This lets you run cargo pgrx schema some_sql_entity_name (like a type or function) and it emits to stdout the correct SQL for you to manually play into your development database to add that SQL entity to your database.

This is super helpful for cases where you add a new UDF and want to get it registered in your already-created extension rather than doing DROP EXTENSION and CREATE EXTENSION.