Rework foreign key constraint introspection on PostgreSQL

[morozov]

The problem with the existing implementation is that it parses the constraint DDL, so the referenced table name is represented as a SQL expression that needs to be parsed (e.g. inventory."products").

As we're switching to object names represented as value objects, I want to avoid the need to parse this SQL and instead initialize an OptionallyQualifiedName from two distinct values (the schema name and table name). This is already the case for all other platforms.

Additionally, the original implementation likely has these bugs:

1. It won't correctly parse referencing or referencing column names containing comma.
2. Prior to 4.3 (Deprecate relying on the current implementation of the database object name parser #6592), if the referenced table schema needs to be quoted (e.g. because it has upper-case letters), the DBAL will fail to parse the quoted qualified name (e.g. "Inventory".products).

Subsequently, this unification will allow to remove some boilerplate schema introspection code.

The new implementation is borrowed from PostgreSQL JDBC Driver.

Note that this rework isn't being ported to 4.3.x because this implementation loses the information about whether or not a given name used in the constraint (e.g. column names or the referenced table name) need to be quoted. If there was an integration test that covered a foreign key constraint with columns in the upper case, it would have failed. In a subsequent PR, I will make all introspected names quoted as they are introspected in their natural case, which should be preserved.