[lib] Introduce SIWEAuthServerCall and use as argument for siweAuth()

Summary:
Introduce the SIWEAuthServerCall type and use as the argument for siweAuth().

We're also updating the return type of the siwe_auth endpoint to boolean for now. The boolean will represent whether or not the nonce, message, and signature in the request are valid and the auth request is "succesful." However, we won't actually register or log in just yet, that will be handled in subsequent diffs (at which point we'll switch from boolean to something like RegisterResponse | LogInResponse).

Depends on D6025

Test Plan: Will be tested implicitly by subsequent diffs... but able to hit endpoint.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6026