Add basic Rust tool chain to GDK

Overview

Adding automation to set up a Rust environment and compiler in GitLab's Development Kit. The intent is to support a basic Rust setup, but not the development of features with Rust at this point. This could be useful for the following use cases:

• A Ruby gem that comes with a native extension written in Rust instead of C, where we cannot download a pre-built platform binary.

Examples

• https://github.com/gjtorikian/commonmarker
• https://github.com/y-crdt/yrb

Proposal

It is not quite clear what is going to be the best way to support Rust, but there are at least two options:

• Add asdf-rust to set up a Rust environment
• Install Rust via rustup

There are a few more considerations, like how to support stable vs. nightly channel, but for the moment I suggest to only support the basic tool chain. This would be a single target (current platform triple, e.g. x86_64-apple-darwin, x86_64-unknown-linux-gnu) and the stable release channel.

cc/ @ashmckenzie