Add collation_schema parameter for schema-qualified PostgreSQL collations

Add collation_schema parameter for schema-qualified PostgreSQL collations

Added a new parameter String.collation_schema (and the corresponding
postgresql.DOMAIN.collation_schema and ColumnOperators.collate.collation_schema),
allowing a PostgreSQL schema-qualified collation name to be specified
explicitly, rather than embedding the schema name within the collation
string itself, which previously rendered incorrectly. As part of this
change, all collation-name rendering across DDL and the collate()
construct now consistently uses the dialect's identifier preparer for
quoting, rather than several separate, inconsistent hand-quoting code
paths; as a side effect, simple lowercase collation names such as
"utf8" are no longer unconditionally quoted in generated DDL.

Fixes: #9693
Change-Id: I6f7214d525d1df44c9049b92820745fc9ad16ce8