Add Rust lang

Goal

Add basic support for a Rust tool chain to GitLab (and therefore GitLab CI). This issue is specifically for CI, the original issue can be found here.

Why

We expect gem's to eventually adopt Rust as a language for native extensions and the continued investment of the Ruby community into Rust bindings and cross compilation already results in existing gems being ported as well (e.g. commonmarker).

MR

There is a Draft MR that need some more work, but could be used as a starting point: !610 (merged)