Implement new signMultiChain method

[esaminu]

• signMulichain(transaction, keyDerivationPath): Promise<void>
• signMultichain will open /sign-multichain route on signer app Add new /sign-multichain route fast-auth-signer#168 and post a message with the interface set on the ticket (transaction to multichain contract, key derivation path)
• Set a listener for a postmessage back from the iframe containing a SignedDelegate that calls the mpc contract's sign method
• Send the SignedDelegate to the relayer which executed the function call to the sign method and returns the signature for the derived key issued by the smart contract