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Add a plugin hook for `setup-py` kwargs  (#10721)

### Problem

Users want to be able to automate some of `setup-py`, such as setting the `version` from a file checked into VCS, or even automatically incrementing the version. Pants itself has a need for this, which we used to achieve through the `pants_setup_py` object.

Using an `object` or `context_aware_object_factory` is inadequate. It does not have access to the Rules API, so cannot do things like safely read from the file system or safely read options from subsystems. It's also hard to get caching right when using `object`.

### Solution

Expose a new hook `SetupKwargsRequest`, which allows users to set whatever kwargs they want through writing a rule that returns `SetupKwargs`. They will have access to the original explicit kwargs, and can also use the Rules API and Target API to do more fancy things like reading from the file system.

These kwargs cannot set certain values like `package_dir` and `packages`. This is because Pants will set those values - the plugin is instead meant to set other metadata like `version` and `long_description`.

### Result

A very simple example plugin:

```python
class DemoSetupKwargsRequest(SetupKwargsRequest):
    @classmethod
    def is_applicable(cls, _: Target) -> bool:
        return True


@rule
def setup_kwargs_plugin(request: DemoSetupKwargsRequest) -> SetupKwargs:
    return SetupKwargs(
        {**request.explicit_kwargs, "plugin_demo": "hello world"}, address=request.target.address
    )

```

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