(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

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)