[WIP] add --python-implementation to pip install

[schlamar]

Summary

Closes #6246

I did some experiments for my feature request.

In theory it works. However overriding implementation without specifying the abi doesn't make sense I would say.

Right now with my changes uv pip install --python-implementation pp --python-version 3.10 would generate the following tags:

pp310-pypy310_pp312-win_amd64
py310-none-win_amd64
py3-none-win_amd64
py39-none-win_amd64
py38-none-win_amd64
py37-none-win_amd64
py36-none-win_amd64
...

I guess adding a --python-abi should be added, too? Otherwise, this feature doesn't makes sense for me.

In pip the following works:

pip install --implementation pp --python-version "3.10" --abi pypy310_pp73 --target test --only-binary=:all: numpy 

About --implementation py. That is some sort of implicit behavior in pip. Maybe there should be a separate flag for this kind of feature in uv? --implementation-agnostic or something like that.

I hope my first draft is going in the right direction and you can provide some feedback.

Test Plan