[keyserver] Persist signedIdentityKeysBlob in keyserver on siwe_auth

Summary:
(Same as D6908, but for siwe_auth endpoint.)

On siwe_auth, we persist signedIdentityKeysBlob in the signed_identity_keys column of the cookies table.

Next up is handling login/register/siwe_auth on the native client. Then will add some validation regex tests just to make things a bit more robust.

Depends on D6908

Test Plan:
siwe_auth succeeds on web and signed_identity_keys is populated as expected when signedIdentityKeysBlob is included in siwe_auth request:

(Observe that both social_proof and signed_identity_keys column are populated w/ same public key)

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6909