Added vector datatype support in Oracle dialect

Added vector datatype support in Oracle dialect

Added new datatype :class:`_oracle.VECTOR` and accompanying DDL and DQL
support to fully support this type for Oracle Database. This change
includes the base :class:`_oracle.VECTOR` type that adds new type-specific
methods ``l2_distance``, ``cosine_distance``, ``inner_product`` as well as
new parameters ``oracle_vector`` for the :class:`.Index` construct,
allowing vector indexes to be configured, and ``oracle_fetch_approximate``
for the :meth:`.Select.fetch` clause.  Pull request courtesy Suraj Shaw.

Fixes: #12317
Closes: #12321
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12321
Pull-request-sha: a72a18a45c85ae7fa50a34e97ac642e16b463b54

Change-Id: I6f3af4623ce439d0820c14582cd129df293f0ba8