[lib][keyserver] Introduce SIWEAuthRequest and siweAuthRequestInputValidator
Summary:
SIWEAuthRequest defines the interface of the siwe_auth endpoint. This interface is validated on the keyserver by siweAuthRequestInputValidator.
NOTE: D5603 included a source field in siweAuthRequestInputValidator, but I don't believe it's necessary or relevant.
Depends on D6024
Test Plan: Tested implicitly by subsequent diffs.
Reviewers: ashoat, tomek, kamil
Reviewed By: ashoat
Subscribers: kamil
Differential Revision: https://phab.comm.dev/D6025