Add uv based solver backend

[jaimergp]

Should be similar to the current pip backend

[jezdez]

Wondering if it's worth separating that into a conda-uv long-term?

[jaimergp]

Torn between split projects or split subcommands. Maybe this repo should become conda-pypi, which exposes conda pip and conda uv subcommands?

[jaimergp]

We'll need these issues closed upstream before we can proceed, since there's no --report flag yet in uv pip install:

• Include a method to create machine readable output astral-sh/uv#3199
• Implement --report for uv pip install astral-sh/uv#1442