Feature Request: Send a unique User-Agent header

[ewdurbin]

When recently debugging an issue with implementation of PEP 639 in PyPI, I noticed that hatch sends a generic header of python-httpx/0.27.0 (or similar based on version of httpx).

I believe by setting a User-Agent header when constructing an index client here, all interactions with PyPI would be a bit more traceable when determine what publishing tool is interacting with the index.

I would reccommend something like hatch/v1.13.0 or potentially including major Python version or critical dependencies like hatch/v1.13.0 CPython/3.13.0 httpx/0.27.2. This will help in debugging any future interop issues.