Support schema-qualified collation in types for PostgreSQL

[dlax]

We add a collation_schema in types supporting collation, along with postgresql.DOMAIN, allowing to create respective elements with a schema-qualified collation name.

Reflection is also handled for both types and domains.

Fix #9693.

[dlax]

notes:

• this is based on #12510 (first 4 commits)
• ~~5th commit (related to reflection caching to avoid JOINs) might be worth considering apart from this PR~~

[dlax]

Rebased (and changed according to the rework of https://github.com/sqlalchemy/sqlalchemy/pull/12510).

[CaselIT]

thanks, sorry for the delay, I'll come to it