Support `matmul` (@) as an optional operator.

Support `matmul` (@) as an optional operator.

Allow custom operator systems to use the @ Python operator (#12479).

### Description
Add a dummy implementation for the  `__matmul__` operator rasing `NotImplementedError` by default.

### Checklist
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This pull request is:

- [ ] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [X] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #12583
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12583
Pull-request-sha: 7e69d23610f39468b24c0a9a1ffdbdab20ae34fb

Change-Id: Ia0d565decd437b940efd3b97478c16d7a0377bc6