Implement the allow propagation on Gateway

[immortal-tofu]

When the Allowed event is triggered, two operations needs to be done by the Coprocessor:

• If the ciphertext associated with the handle has not been bootstrapped, the SNS worker needs to compute the 128 bits bootstrap and push the ciphertext and bootstrapped ciphertext to the Gateway. This issue is handled by Implement add_ciphertexts workflow #374
• In any case, the authorization needs to be added to the Gateway. For this, the coprocessor needs to emit a transaction to the Gateway, passing the handle, the address and the chainId. See tech specs for more information about the workflow.