Support for injecting custom toolchain features (approach injecting legacy features with example)

[limdor]

Hi, this is just a draft how a proposal could look like to implement #517 on the legacy toolchain approach defining legacy toolchain features. Because there is no rule for that, rules for the providers should be created.

Another approach would be to support the injection of toolchain features in a legacy toolchain but require that such injected feature should be a rule-base feature and then the implementation just calls https://github.com/bazelbuild/rules_cc/blob/main/cc/toolchains/impl/legacy_converter.bzl#L105 before injecting it.

Let me know your opinion

If you want to try it out just run:

bazel build //examples/inject_extra_features:main --extra_toolchains=//examples/inject_extra_features/toolchain:my_linux_toolchain --toolchain_resolution_debug=.* --subcommands=pretty_print --features=foo

[armandomontanez]

Because there is no rule for that, rules for the providers should be created.

This is actually how the rules-based toolchain rules started. The returned providers diverged over time because we wanted to enrich the experience. It'd be best to just go all-in on the existing toolchain rules (like in #524) rather than introducing a new derivative.