feature request: repository_ctx.copy

### Description of the problem / feature request:

I'd like a `repository_ctx.copy` function, to copy file. I'm currently using `repository_ctx.template`, but that's an abuse of the API and it does not work with binariy files: #11857

### Feature requests: what underlying problem are you trying to solve with this feature?

Coping file in repository rules. I can use `execute(["cp", ...)`, but a `copy` api can be more convenient and more robust as it does not depend on the availability of a `cp` command.

Start by tracing the existing repository_ctx.template implementation and the repository-rules context it uses. Confirm how binary files are currently handled, then define the copy API's behavior and tests around copying files without relying on an external cp command.

Written by the indexing model from the issue text.