Add pg DOMAIN type reflection

Add pg DOMAIN type reflection

The PostgreSQL dialect now returns :class:`_postgresql.DOMAIN` instances
when reflecting a column that has a domain as type.
Previously the domain data type was returned instead.
As part of this change, the domain reflection was improved to also
return the collation of the text types.

Fixes: #10693
Closes: #10729
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10729
Pull-request-sha: adac164d191138265ecd64a28be91254a53a9c25

Change-Id: I8730840de2e7e9649067191430eefa086bcf5e7b