Fixed JSONB path_match and path_exists operators to use correct type coercion

Fixed JSONB path_match and path_exists operators to use correct type coercion

Fixed issue where PostgreSQL JSONB operators
:meth:`_postgresql.JSONB.Comparator.path_match` and
:meth:`_postgresql.JSONB.Comparator.path_exists` were applying incorrect
``VARCHAR`` casts to the right-hand side operand when used with newer
PostgreSQL drivers such as psycopg. The operators now indicate the
right-hand type as ``JSONPATH``, which currently results in no casting
taking place, but is also compatible with explicit casts if the
implementation were require it at a later point.

Fixes: #13059
Change-Id: I8e1a58361456f7efabf4940339cb5ce2c5a1d5f9