[DRAFT] Support tokio::process::command.

[nickpdemarco]

After hitting this issue I tried to add basic support for tokio::process::command. It requires a new trait, AsyncOutputAssertExt in order to get the following to compile:

    tokio::process::Command::cargo_bin("my-bin")
        .expect("")
        .assert()
        .await
        .success();

I have not yet done the due diligence to confirm this covers all reasonable use cases. Raising this PR for visibility and to ask for some guidance on how to get this production-ready. Documentation TODO.

[epage]

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