feat: add support for git source dependencies in pixi build

[nichmor]

Overview

Adding support of git dependencies for pixi build.
New and important changes:

• adding subdirectory in PixiSpec. This means that user can write this in pixi.toml:
  boost-check = { git = "https://github.com/nichmor/pixi-build-examples", subdirectory="something" }
  Other supported git references:
  boost-check = { git = "https://ghp_yourpersonal_token@github.com/nichmor/pixi-build-examples.git", branch = "main"}
boost-check = { git = "ssh://git@github.com/nichmor/pixi-build-examples", branch = "fix-backend" }
  or any URLs with user and password

In the screenshot, with the green color I highlighted this field.

• Adding pixi_git crate. This is copied mostly from uv-git and trying to keep minimum additional changes.
  It expose GitUrl ( which is not aware of subdirectory, just a url from git perspective ) and a GitResolver , that can receive GitUrl and fetch it somewhere in the cache and return the path to fetched repository.
  In screenshot we can see a small overview of it:
  

• on pixi_build, I changed PinnedGitSpec. It now contains a new field of PinnedGitCheckout type which represent a solved checkout of some source git spec. This, later, will be transformed into a LockedUrl and saved in pixi.lock.
  LockedUrl redact all the credentails, contains a mandatory git+ suffix and have exact commit in the fragments.
  In the attached screenshot, with green I highlighted the new types and how they behave.
  

• get_update_lock_file_and_prefix now also contain store_credentials_from_project function. This function is responsible for loading all the credentials from pixi.toml, and apply them to the locked urls ( because they are redacted and don't contain anymore any creds)

My vision is that by using LockedGitUrl , we can also represent PyPi types, which can be latter transformed back from lock file into some PyPi type. Also, rattler-build or pixi-build-backends can reuse pixi_git Giturl for they needs when solving some dependencies ( as they are unaware about subdirectories ) and can be constructed from the Urls.

Blocked things that needs to be merged before this PR:

• pixi-build-backends should support new subdirectory in the manifest

Missing things that will be added in following PR's:

• pixi add should also support adding git url
• we should add progress reporter to pixi_git.
• some integration tests/unit tests.

[tdejager]

short_sha == long_sha if it is the same :)

[nichmor]

we always record requested rev, so if user previously short sha we will check against it

[tdejager]

Great work thank you 🥂 !