Add a "create contract code" endpoint to the v3 definition

[zxq9]

In the course of developing the Vanillae library it became evident that creation of contract call data is a bit complex and difficult, and not handled any standard way anywhere. The short term solution was to simply write out the routines necessary to create the call data actually in Vanillae itself, mostly by mimicing what the node does internally. This was easy because the Vanillae's first language binding was for Erlang and the node is written in Erlang, but this is not a good general solution for other languages later on. (example: https://github.com/aeternity/Vanillae/blob/21040bf4b868194e1056715948b3e14e6ae749c2/bindings/erlang/src/vanillae.erl#L1205-L1340 )

The easiest way to fix this is to create a new endpoint on the node that can accept a contract call definition (function name + args), an ACI, and the remaining transaction details (gas, fee, gas price, amount, etc.) and simply return the final contract call data. This should be relatively trivial given that the code to do this is already in the node, just not hooked up to any endpoint at the moment.

[hanssv]

This is not trivial, the code you refer to is only a test dependency to the node (and for a good reason).