"switched" command runner

[tdyas]

Introduce SwitchedCommandRunner which uses a predicate to choose between two command runners. Then use SwitchedCommandRunner to switch between the docker::CommandRunner and whatever other local command runner is configured.

This allows docker::CommandRunner to no longer know about any other command runners, thereby re-introducing better encapsulation to docker::CommandRunner.

Closes #16788

[Eric-Arellano]

Thanks!

[stuhood]

The nailgun command runner should be able to use this as well:

pants/src/rust/engine/process_execution/src/nailgun/mod.rs

Lines 79 to 91 in c702901

	///
	/// A command runner that can run local requests under nailgun.
	///
	/// It should only be invoked with local requests.
	/// It will read a flag marking an `Process` as nailgunnable.
	/// If that flag is set, it will connect to a running nailgun server and run the command there.
	/// Otherwise, it will just delegate to the regular local runner.
	///
	pub struct CommandRunner {
	inner: super::local::CommandRunner,
	nailgun_pool: NailgunPool,
	executor: Executor,
	}

[tdyas]

I switched this to using generic types instead of Box<dyn CommandRunner>. Then I added an impl of CommandRunner for Box<T> where T: CommandRunner.

[tdyas]

The nailgun command runner should be able to use this as well:

Will look into a follow-up.