Replace relayer with Identity.isValidSignature in Ethereum adapter

[elpiel]

In V5 with new Identity contract we should not call relayer anymore instead we should use the Identity contract's isValidSignature.

• Create a test using Payload.identity for authentication
• Update Snapshot to include Identity contract (see [CI] Use a snapshot for Ethereum adapter testing #403) Not needed, we can deploy the identity when needed.

Pseudo code old:

const signer = ecrecover(hash(token)) // whoever signed the message
if (isIdenity) return getPrivilegesFromRelayer(identityAddr)[signer] ? identityAddr : null
else return signer

Pseudo code new:

const signer = ecrecover(hash(token)) // whoever signed the message
if (isIdentity) return new Identity(identityAddr).isValidSignature(sig) ? identityAddr : null
else return signer

[Ivshti]

@elpiel Correct, we an just use a signature for the EIP 1271 spec and verify this with isValidSignature