Cross msg: Arbitrary send-cross function

[adlrocha]

The IPC agent only supports at the moment sending cross messages through the basic primitives of fund and release, which means that cross messages need to be sent from one subnet to another one level at a time, requiring users to perform this individual operations.

The agent should abstract from this low level primitives, and expose a method that allows to send arbitrary messages to any subnet. The agent will decompose under the hood this request into a set of low-level fund, release, propagate operations until the messages is executed in its destination. Thus, the agent is responsible for tracking when one step from one subnet to the next one is committed, and triggering the subsequent operation to the right subnet to fulfill the request.

Related: #76