Add Default for conda-pypi-map in Config #6856

Problem description

#6333 added configuration options for the conda-pypi-map in the pixi.toml manifest.

Add a section default-conda-pypi-map to the config.toml that allows organisations to set site-wide defaults for this section. It works, much like default-channels, by adding the conda-pypi-map section on pixi init. pixi init --conda-pypi-map <CONDA_PYPI_MAP> already exists and will override the default behaviour.

[default-conda-pypi-map]
conda-forge = { location = "https://example.com/mapping.json", mapping-mode = "replace" }
internal = false

In addition (maybe another PR), it would be useful to be able to set a default for channels not explicitly listed by setting

[default-pypi-conda-map]
conda-forge = { location = "https://example.com/mapping.json", mapping-mode = "replace" }
"*" = false  # overrides unspecified channels to use no mapping, maybe also supported in the manifest?

I have a working branch at main...FHTMitchell:pixi:fm/default-conda-pypi-map