Add upgrade subcommand

[jeffparsons]

Installs the latest version of cargo-component and upgrade to the corresponding version of cargo-component-bindings.

Follow-up to #167 (comment).

Before merge this needs at least:

• Review (duh)
• Squash + meaningful commit message

In particular I would like to draw the reviewer's attention to:

• Design decision to unconditionally cargo-install and then exec delegate to either the existing binary or newly installed binary (unknown to us) for the rest of the job, as originally mentioned here: Refactor bindings generation. #167 (comment).
• cargo-install part is currently completely untested. (Though it Works For Me™.) For now I've gated it behind #[cfg(test)] so I can still at least have the tests delegate to a child process to exercise as much of that code path as practicable. Any better ideas?
• I've added a boolean function parameter. Does that lead to a paddlin' around here? 😅 I'm happy to make it an enum with meaningful variant names if that's the preferred thing here.
• I haven't added a test that it actually updates all crates in the workspace. (Though again it Works For Me™.) Should I do that?

[jeffparsons]

@peterhuene I think this is basically what you suggested. PTAL? 🙏

[jeffparsons]

Oh yeah, Windows. Forgot about that. 😅

I'll fix.

[peterhuene]

@jeffparsons thanks for the PR! I'll review this shortly.

[peterhuene]

This looks really fantastic, thanks for taking this on!

I just have one minor suggestion to the message that gets printed when projects are up-to-date.

[jeffparsons]

Good call; I've applied that change and rebased.

Thanks for the review! 💖

[peterhuene]

Thanks again for the PR!