Support `uv` as part of `rules_python`

Just creating this as a placeholder for discussions and documenting some thoughts/findings. `rules_uv` is doing its job well and replicating the functionality in `rules_python` would be duplicate effort. However, `rules_uv` does not support `WORKSPACE` installations (at least the [releases] don't advertise such support) and `rules_python` needs to support `WORKSPACE` installations.

[releases]: https://github.com/theoremlp/rules_uv/releases

As a result I see the following options:
1. Do not support `uv`. This has a drawback of longer compilation times with the included `pip-compile` tooling. `uv` is just a binary and could in theory be later reused for common operations with whl packages.
2. Support `uv` only on `bzlmod` by including `rules_uv` as a dependency. This would require us to expose `rules_uv` via `@python_versions` or something similar so that we don't have loads that load `rules_uv` within the main `//python/*.bzl` files to avoid breaking `WORKSPACE` users. However this would not allow us to use it for our own example testing because they should produce the same output with and without bzlmod. Since we cannot dog-food `rules_uv`, it makes it a hard proposition.
3. Write the `WORKSPACE` `deps.bzl` implementation for `rules_uv` so that we can depend on it.
4. Support `uv` by copying/reimplementing the `uv` support as a separate rule/macro whilst not relying on `rules_uv`. The drawback is that we would have to reimplement parts of it, but the benefit would be that the extra tests that we would need to have would be next to the implementation. We could also provide Windows support in this way as `rules_uv` does not support Windows yet due to relying on `bash` scripting for `pip_compile`.

Current status:
- [x] `uv pip compile`, works with custom authentication helpers.
- [x] `uv lock`, works with custom authentication helpers.
- [x] Parse `uv.lock` and support `requirements.txt` feature subset, works with bazel downloader, no sdist support.
- [x] Downloading and customizing downloaded `uv` versions.

Start by reviewing the current checklist and the four implementation options, then inspect the //python/*.bzl entry points, WORKSPACE deps.bzl support, bzlmod integration, and the rules_uv releases. Compare the existing uv.lock and requirements.txt handling and example-test expectations. Done is not defined until the project selects an approach and specifies the required WORKSPACE and bzlmod behavior.

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