[runtime] Add `tokio` Variant That Uses Existing Instance

[patrick-ogrady]

runtime::tokio assumes that it has control over tokio:

monorepo/runtime/src/tokio/runtime.rs

Lines 250 to 255 in 0fe36db

	let runtime = Builder::new_multi_thread()
	.worker_threads(self.cfg.worker_threads)
	.max_blocking_threads(self.cfg.max_blocking_threads)
	.enable_all()
	.build()
	.expect("failed to create Tokio runtime");

We should add a variant that doesn't require access directly to the runtime and instead use tokio::spawn (etc) to interact with an externally available tokio instance.