Support `target.x.pypi-options` (and `feature.y.target.x.pypi-options`)

[hameerabbasi]

Problem description

After #4458 was implemented in #6178, this suggestion has become more useful.

Consider the following partial pixi.toml, which demonstrates the utility of this feature.

[workspace]
platforms = [
    { name = "linux-cuda-13", ... },
    { name = "linux-cuda-12", ... },
    { name = "linux-cpu", ... },
    { name = "win-cuda-13", ... },
    { name = "win-cuda-12", ... },
    { name = "win-cpu", ... },
    { name = "macos-cpu", ... },
]

[target.*-cuda-13.pypi-options]
extra-index-urls = ["https://download.pytorch.org/whl/cu130"]

[target.*-cuda-12.pypi-options]
extra-index-urls = ["https://download.pytorch.org/whl/cu126"]

[target.*-cpu.pypi-options]
extra-index-urls = ["https://download.pytorch.org/whl/cpu"]