support BLANK_SCHEMA, RETAIN_SCHEMA FK schema on copy

support BLANK_SCHEMA, RETAIN_SCHEMA FK schema on copy

Added support so that the :paramref:`.Table.tometadata.referred_schema_fn`
callable passed to :meth:`.Table.to_metadata` may return the value
:data:`.BLANK_SCHEMA` to indicate that the referenced foreign key should be
reset to None. The :data.`RETAIN_SCHEMA` symbol may also be returned from
this function to indicate "no change", which will behave the same as
``None`` currently does which also indicates no change.

Fixes: #7860
Change-Id: I82a45988d534295d8356453f68001b21d4ff706d