support for rust-toolchain.toml file

[smallstepman]

• detect if rust-toolchain or rust-toolchain.toml files are present at the root dir of the repo https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
• install rust version, targets, components
• add option to set them as defaults

[dtolnay]

I would prefer not to add this in this project.