Reflect schema-qualified PostgreSQL collations for columns and DOMAIN

Reflect schema-qualified PostgreSQL collations for columns and DOMAIN

The PostgreSQL dialect now reflects the schema of a schema-qualified
column or DOMAIN collation, populating the collation_schema parameter
added to String and postgresql.DOMAIN so that reflected DDL round-trips
exactly. The schema is omitted from the reflected value when the
collation is visible on the current search_path without qualification.

Fixes: #6511
Change-Id: Id9c7016659fb0ce5b83d6be5ad172811cea7dd14