feat: add --path to the pixi add command

[ruben-arts]

Description

This PR add pixi add --path.

It works for both conda and pypi dependencies.

If you use it for conda dependencies and the preview = ["pixi-build"] flag doesn't exist you'll get an interactive request whether you want to add it.

It accepts pixi add --path /bla/package/pixi.toml package-name but will save it as package-name = { path = "../package" } as absolute paths don't work nicely in the manifest.

How Has This Been Tested?

I've tested it by adding some local package on my machine to a new pixi project and I couldn't break it.
e.g.:

pixi add --path ~/dev/pixi pixi
pixi add --path ~/dev/vinca vinca --pypi
pixi add --path ~/dev/ros_ws/src/common/package.xml ros-jazzy-common

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Codex

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[Hofer-Julian]

I don't get this sentence