Add type annotations for Function.filter

Add type annotations for Function.filter

This includes all methods / properties on the returned FunctionFilter
object.

This contributes towards #6810

This pull request is:

- [x] 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.
- [ ] 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.

Closes: #10643
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10643
Pull-request-sha: 6137b7b995b6ea0bd4e4195c5693d2312fa26639

Change-Id: I2af1af7617d0cd3fd30b262d36ff982464bac011
(cherry picked from commit 52452ec39d18567126673eeef4cf0dd12039043b)