Support third-party exercises

[mo8it]

It would be awesome if we could let people create exercises that would work with the Rustlings program.

After doing the built-in Rustlings exercises, people can navigate the ecosystem and try some exercises by clap for example!
The third-party (clap in this example) would provide a directory similar to the exercises directory and a file similar to the info.toml file in Rustlings. Then the user can use the watch mode of Rustlings to navigate through the exercises and get feedback and hints.

We could also use this to outsource things like providing translations #1627. There would be a repository with Rustlings exercises in French for example and users can decide to use that repository instead of the official one in English.