PostgreSQL: Support schema-qualified operator classes in CREATE INDEX

[dddenis]

This PR adds support for schema-qualified operator classes in PostgreSQL CREATE INDEX statements. Previously, the parser only accepted simple identifiers for operator classes (e.g., vector_cosine_ops). With this change, schema-qualified names are now supported (e.g., public.vector_cosine_ops).

The following SQL is now correctly parsed:

CREATE INDEX my_index ON my_table USING HNSW (embedding public.vector_cosine_ops);