Sample contract leveraging sendCross for on-chain interaction with IPC

[adlrocha]

SendCross is a method exposed by the gateway for contracts to be able to trigger cross-net messages as a side-effect from their operation. This hasn't yet been tested. Ideally we should:

• Write a simple contract that when a function is called triggers a cross-net message to some subnet. This can be initially just the sending of funds to the parent or child (for instance).
• Write a tutorial of how this can be done so developers can start using this feature.

In the process, it would be great if we could deploy two child subnets (/r123/A and /r123/B), and we could also test the propagation of messages through propagate. In the future we will have to improve the IPC agent so with a single command, the IPC agent can orchestrate itself the message propagation until it reaches the destination.

[dnkolegov]

There is an example of such a contract here - https://github.com/consensus-shipyard/ipc-solidity-actors/tree/dev/examples/contracts