Provide a mechanism for disabling toolchain registration when using bzlmod.

[konkers]

Some projects want to vendor and tightly control their toolchains. With rules_rust always registering toolchains when used through blzmod, it is easy to misconfigure a project's toolchain configuration and accidentally fallback on the rules_rust toolchains without knowing. Providing a way to explicitly disable the rules_rust toolchains would turn this case into and error instead of a silent fallback.

[konkers]

#2819 Implements a mechanism for inhibiting the registration of default rust toolchains. It does not address:

• proto toolchain
• prost toolchain
• bindgen toolchain
• host_tools repository
• cargo_bazel_bootstrap repository

proto, prost, and bindgen toolchains are directly referenced instead of going through an extension. They would probably need to follow the same pattern as rust_toolchains to allow for disabling their registration.

The host_tools and cargo_bazel_bootstrap repositories are a bit trickier as rules_rust references these repos internally.