[keyserver] Return generated nonce in siweNonceResponder

Summary:

• return nonce generated from siwe:generateNonce
• input validation with tcomb
• introduce SIWENonceRequest and SIWENonceResponse types (to appease flow)

Again nothing very interesting, just necessary scaffolding.

Depends on D5884

Test Plan:
Send a malformed request and make sure the keyserver throws.
Send a correctly formed request and make sure the keyserver validates input:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D5885