[keyserver] Add isValidSIWENonce check to landingResponder

Summary:
Addresses feedback from: https://phab.comm.dev/D5977#180158

If the siweNonce header is set and it's not valid, we immediately return a 400 and return.

If the siweNonce header is empty or it is set and valid, we proceed with the request as usual.

Test Plan:

1. Tested isValidSIWENonce regex with some simple unit tests.
2. Made sure landing generally continues to work as expected on desktop (no nonce).
3. Made sure landing/SIWE continues to work as expected on native.
4. Messed up the nonce in the request to landing/SIWE (hardcoded to some gibberish) and ensured that we got a 400 in response:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6022