Use auto-generated code/ABI for marshalling Packet in e2e fixtures

[gjermundgaraba]

The Packet that is marshalled in the e2e fixture is currently manually created by creating a new ABI type. This would cause issues if Packet struct ever changes (and might only give errors later when the fixtures are used in the solidity tests).

The ideal solution here would be to use something that is generated from the solidity code. Perhaps there is a way to use the ABI from ICS26Router to get the struct from there.

Reference to the discussion: #44 (comment)