Draft: Add Rust as one of the default plugins.

What does this merge request do and why?

Adds Rust as one of our default plugins for gdk. This change does not require Rust to be installed right now (or a specific default version), but allows us to eventually add rust as a dependency to the gitlab repo. This is part of the effort described here gitlab#381569 (closed).

Fixes #1588 (closed)

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.