Fetch verify key locally rather than trying to do so over federation if origin...

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