[keyserver] Introduce input id conversion

Summary:
Modifies the validateInput function to also convert the id schema (conversion is locked behind a flag for now)

• Now validateInput asserts the flow type of the input after validation which allows us to change any to mixed in responder input and remove the cast
• In two places in user-responders validateInput was called with null validator and input. To simplify the validateInput function generic typing I've replaced them with direct calls to verifyClientSupported, which is what was happening anyways

Depends on D7713

Test Plan: Test web app and ios emulator and check for any server errors.

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7714