Read the Rust version from rust-toolchain.toml in rust.toolchain

[u9g]

A repository that has a rust-toolchain.toml for rustup and cargo has to write the same version a second time in MODULE.bazel, and nothing keeps the two in step (#2753). This is the toml.bzl version of #3792 that @UebelAndre asked for there.

rust.toolchain(rust_toolchain_file = "//:rust-toolchain.toml") reads the file with toml.bzl, which is a new bazel_dep:

• channel becomes versions. It must be an exact release (1.85.0, nightly-2025-01-01, beta-2025-01-01): rules_rust downloads a fixed release, and resolving stable or 1.85 to one is something rustup does with the network, so those fail with a message saying so.
• targets are added to extra_target_triples.
• rustc-dev in components enables dev_components.
• Setting versions as well is an error, so neither attribute silently does nothing.

module_ctx.read watches the file, so editing it re-evaluates the extension.

Tested with the unit tests in test/unit/rust_toolchain_file, and by pointing examples/hello_world_no_cargo at a rust-toolchain.toml (included, so CI keeps exercising the attribute): with channel = "1.97.0" @rust_toolchains registers rust_analyzer_1.97.0 and only stable toolchains, channel = "1.98" fails with the exact-release message, and adding versions fails with the both-set message.

Fixes #2753