Merge #7055

Merge #7055

7055: Install rustfmt if needed during formatting request r=matklad a=kinnison

In order to fix #6052 and to fix #4249 this PR installs `rustfmt` using `rustup` if `rustfmt --help` fails.

In theory we ought to memoise the attempts (both the `--help` and the installation) so that we don't keep retrying something which will fail (e.g. if nightly is missing rustfmt), but this is a first-pass for opinions.

In order to make it possible to notify the user of what happened, I added a facility for `GlobalStateSnapshot` to send *messages* to the client indicating what happened.  There may be a cleaner way to do this as well but I wasn't sure exactly what might be best given this is my first time in this codebase.

It may be worth, longer term, working with `rustup` to provide a way to detect a missing component binary since `rustup` returns `1` if `rustfmt` is not installed, which is not ideal.

Co-authored-by: Daniel Silverstone <dsilvers@digital-scurf.org>