feat: add support for exercises that depend on crates

[Ben2917]

Based on my suggestion in this issue, I've added support for exercises that rely on crates. I've also added a simple mockall exercise as an example (I'm happy to add more of these if the feature is accepted).

I think this would be a worthwhile addition to rustlings. It would be great if in the future crate authors have the option to push some simple exercises to get people started with their crate.

Please let me know what you think, I'm happy to make some updates. Thanks!