Implement msgflo-register-foreign in Rust

[bergie]

The msgflo-register-foreign tool from MsgFlo itself is nice for adding non-MsgFlo-aware participants to a MsgFlo graph.

However, the downside is that since it runs as a Node.js process, it is quite heavy on memory. We should implement a compatible CLI tool in Rust for memory-constrained environments.

The Node.js source code is at https://github.com/msgflo/msgflo/blob/master/src/utils/register-foreign.coffee