Add a library that defines a `Label` class in python

# 🚀 feature request

### Description

I would love `rules_python` to expose a library similar to [@rules_rust//util/label](https://github.com/bazelbuild/rules_rust/blob/0.32.0/util/label/label.rs) which defines a validated representation of a [Label](https://bazel.build/rules/lib/builtins/Label) for users to use in their code.

It'd be fantastic if the interface was a class (`Label`) that did validation on `__init__`.

Start by comparing the requested interface with @rules_rust//util/label and the Bazel Label documentation linked in the issue. Define the scope of validation and where the new Python library belongs, then verify that it exposes a Label class whose constructor validates its input.

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