monitor: Move commands to a separated package

[ktock]

Following-up #1640 (comment)

From @jedevc 's comment:

The commands in the monitor are getting long - we really need to split these up, out of a switch statement, and ideally into
something a bit more dynamic. Something like implementations of a monitor.Command interface?

As inspiration:

type Command interface {
    // executes the command
    Exec(args []string /* anything else we might need*/)

    // returns command information
    Info() Info
}

type Info struct {
    // a usage string, e.g. "rollback [--init] [entrypoint] [cmd...]"
    Usage string

    // a short description, e.g. "re-runs the interactive container with initial rootfs contents"
    Description string

    // a longer description, describes in detail what the command does, the
    // behavior of each arg, etc.
    Help string
}

This commit moves monitor commands to monior/commands package. Commands still need access to the monitor object and buildx controller so this commit enables this via Monitor interface stored in monitor/types.

[jedevc]

Just a couple of initial thoughts, haven't done a proper deep dive into the command implementations yet, will try and get to that tomorrow ❤️

[jedevc]

Could you rebase onto master since #1737 merged?

[jedevc]

This is good now IMO 🎉

I think we can do some future tidy-ups, but this is a significant improvement from the big switch statement 🎉 🎉

Thanks!