[lib][keyserver] Introduce basic scaffolding for siwe_nonce endpoint

Summary:
Scaffolding I've been using to hit the siwe_nonce endpoint from the native client. There's nothing interesting here... just pushing stuff up from local copy.

Depends on D5883

Test Plan:
Able to hit endpoint from native client:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D5884