Add support to inject cc_feature into legacy toolchains

[limdor]

This adds a parameter to unix cc_toolchain_config and windows cc_toolchain_config to be able to inject a cc_feature. This allows a user to inject a feature (for example for custom warnings) without having to reimplement the full toolchain.

Resolves #517

[limdor]

I added to 0538f26 to see if the rest of the tests were passing. I believe that this is not wanted but I would ask for advice. The problem we have here is that the bzl file ends up in a generated repository. AFAIK there is no way or it is not a good practice to allow visibility to a specific named repository.

[armandomontanez]

Re: visibility

I'd expect adding "@local_config_cc/...", should do what you want.

[limdor]

@armandomontanez it does not work. It does not seem to be allowed to use the @ notation

Error in visibility: invalid package name '@local_config_cc//...': must start with '//' or be 'public' or 'private'

[limdor]

@armandomontanez could you provide guidance on which direction you would like me to take? For the providers file, if the direction is to make it public I created #540
For the file in impl, it is a bit more tricky. I could create another bzl file that only expose the interested conversion function, but I would need guidance on where to put such bzl file and how to call it.

[limdor]

@armandomontanez can you take a look again? With the changes that I incorporated only the required parts are exposed.