Add API so allow for a select-like mechanism using the ROUTER

[gterzian]

I'm running into this at servo/servo#26906

Essentially I have a route, which needs to select on two channels. I do this currently simply by having the same IPC channel shared in two different places, one being actually in the same process.

So you end-up with a message struct like:

pub enum BodyChunkRequest {
    /// This is over IPC
    Chunk,
    /// Signal the stream is done.
    /// Signal the stream is done(sent from script to script).
    Done,
    /// Signal the stream has errored(sent from script to script).
    Error,
}

Note the "script to script" messages. So essentially, some messages are received over IPC, which is the "normal" use-case for the route, but then there are also a few signals that are received from a thread on the same process.

Come to think of it, this is really about being able to add a channel, probably a normal crossbeam channel, to send messages to the route from the same process where it has been setup.