If public key passed as header (siwePrimaryIdentityPublicKey), we'll include it in the SIWE message statement.
For the timebeing if a siwePrimaryIdentityPublicKey is NOT being passed in as header, we'll fallback to the previous SIWE message statement without the public key. Will go back and enforce existence of siwePrimaryIdentityPublicKey (with necessary error handling, error messages, etc) later in the stack.
Depends on D6131
