Fetch verify key locally rather than trying to do so over federation if origin and host are the same. (#11129)

* add tests for fetching key locally

* add logic to check if origin server is same as host and fetch verify key locally rather than over federation

* add changelog

* slight refactor, add docstring, change changelog entry

* Make changelog entry one line

* remove verify_json_locally and push locality check to process_request, add function process_request_locally

* remove leftover code reference

* refactor to add common call to 'verify_json and associated handling code

* add type hint to process_json

* add some docstrings + very slight refactor