feat(build): url source

[remimimimimi]

Description

Extended url specs with directory subdir and finished plumbing for url out-of-tree builds.

How Has This Been Tested?

I added example similar to example with git and run pixi lock and pixi build. Build succeeded and I checked that computed sha256 is equivalent to the one computed manually. In particular I ran

wget https://github.com/prefix-dev/pixi-build-testsuite/archive/b5dc4ae6ee844045ea63af8d0de8eaa585207294.zip
cat b5dc4ae6ee844045ea63af8d0de8eaa585207294.zip | sha256sum

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.

[remimimimimi]

Should not be merged before #4875.