Support for `tokio::process::Command`

[nickpdemarco]

After hitting this issue I tried to add basic support for tokio::process::command in #223. I was asked to raise an issue here to discuss whether this is desirable and, if so, how best to proceed.

[epage]

Looks like all of the building blocks are present for tokio::process::command to work, this would just be about providing convenience functions for it.

My own personal biases

• I tend to keep things small, simple by not providing integrations with other libraries in my crates
• I tend to avoid touching async as I have no use cases for it which both makes me shy away from async contributions and makes it harder to evaluate them
• I've been tempted to deprecate assert_cmd in favor of snapbox but then we'll just have the conversation over there :)

I've been going back and forth on whether to move forward with this.