[web] Pass signedIdentityKeysBlob to keyserver on siwe_auth

Summary:
Include signedIdentityKeysBlob in siwe_auth request and update siweAuthRequestInputValidator to accept signedIdentityKeysBlob. Right now we don't do anything with the signedIdentityKeysBlob, but it's being passed to keyserver from web when hitting both login and siwe_auth endpoints.

Next up is

1. Making sure that signedIdentityKeysBlob is valid (regex checks)
2. Pulling out primary.ed25519 and verifying the payload signature (maybe an argument for leaving primaryIdentityPublicKey as part of the login request and maybe adding it to siwe_auth?)
3. Stringifying the entire signedIdentityKeysBlob and storing it in the serverDB (going to do a migration to remove existing stuff first).

Depends on D6895

Test Plan:
Set breakpoint in siweAuthResponder and observed that signedIdentityKeysBlob "made it through" and validator successfully validated request:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6896