feat: add pypi index

[nichmor]

Description

The stretch goal is to add custom indexes for pixi like here: https://docs.astral.sh/uv/configuration/indexes/
However, In this PR we add only pinning https://docs.astral.sh/uv/configuration/indexes/#pinning-a-package-to-an-index

A small example of this would be would be:

[project]
authors = ["nichmor <[email protected]>"]
channels = ["conda-forge"]
description = "Add a short description here"
name = "custom_pypi"
platforms = ["osx-arm64"]
version = "0.1.0"

[tasks]

[dependencies]
python = "*"


[pypi-dependencies]
pytorch ={ version = "*", index = "https://download.pytorch.org/whl/cu118" }

We want to iterate on this further and allow the user to specify indexes in the manifest and pin to them by name. This will be done in subsequent PR's.

Tests

There are two automated tests that have been added:

1. Test of pinning logic in an integration test.
2. Test of pinning to pytorch specifically, so we can be sure that this works.

[ruben-arts]

Looks good, could you add it in the docs/reference/project_configuration.md?

[ruben-arts]

I think it's good to also add torch here, it shouldn't be to slow as it's only solving and it's the main usecase for this feature.

[tdejager]

I've added this in different test, I think its better to isolate this one :)

[tdejager]

Something is broken, we need to have this fix first: #2501

[tdejager]

This should all be good now, the failing test is unrelated.