Add type annotations to `postgresql.json`

Add type annotations to `postgresql.json`

(Same as https://github.com/sqlalchemy/sqlalchemy/pull/12384, but for `json`.)

### 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.

Related to #6810

**Have a nice day!**

Closes: #12391
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12391
Pull-request-sha: 0a43724f1737a4519629a13e2d6bf33f7aecb9ac

Change-Id: I2a0e88effccf351de7fa72389ee646532ce9cf69
(cherry picked from commit c7f4e8b9370487135777677eaf4d8992825c24aa)