Config option to inhibit 3PID errors on /requestToken

Adds a request_token_inhibit_errors configuration flag (disabled by
default) which, if enabled, change the behaviour of all /requestToken
endpoints so that they return a 200 and a fake sid if the 3PID was/was
not found associated with an account (depending on the endpoint),
instead of an error.

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>