feat: pixi add git source dependency

[nichmor]

closes: #2795

Overview

Extend pixi add to support git source dependencies.
pixi add --git --branch/--rev/--tag --subdir

To test it, you can cd examples/pixi-build/cpp-sdl and run:
pixi add --git https://github.com/wolfv/pixi-build-examples --branch main --subdir boost-check boost-check
or select a different commit/tag.

Some implementation details/choices:

• Manifest.add_dependency now accepts PixiSpec and PackageName instead of Matchspec. This allows in my opinion to be more flexible when adding pixi specs into manifest, as PixiSpec can represent SourceSpec/BinSpec and move the responsibility of creating it to the caller side rather than manifest side. It also saves manifest to not accept ChannelConfig

• project.update_dependencies now accept also SourceSpec.

We also support adding git for pypi:

pixi add --git https://github.com/mahmoud/boltons.git boltons --pypi

[ruben-arts]

Hey this works great!

Could you add some documentation of the CLI and get it working for pypi aswell?

[ruben-arts]

Thanks! Looks good to me.