Add support for toolchain version from `rust-toolchain.toml` file

[jrandolf-google]

The toolchain file is the de-facto way to pin the toolchain. Why not have rules_rust also read from it!

[illicitonion]

From https://bazelbuild.slack.com/archives/CSV56UT0F/p1716902401450119

It absolutely would be possible, and I'd gladly review a PR to make it so!

I imagine you'd want to roughly:

rules_rust/rust/repositories.bzl

Line 378 in a92de54

version = ctx.attr.version

- make this version attr either optional, or take a sentinel value like "FROM_RUST_TOOLCHAIN.TOML", and in this line check for that and ctx.read(ctx.workspace_root / "rust-toolchain.toml") if it exists instead.

cc @ParkMyCar who was maybe interested in giving this a go

[ParkMyCar]

I do have an interest! Might be able to get to it later this week, but if someone else wants to give it a go, don't block on me