- [x] Have you followed the guidelines for contributing? - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [x] Have you successfully run `make lint && make test`? - [ ] Have you added an entry to the changelog (`docs/reference/changelog.rst`)? --- Implementation of ST178: https://docs.google.com/document/d/1pQtEWU_q1kR7H0sf5StmISvhmhQ2-9MBM9ciw3uG88o/edit?tab=t.0 This adds experimental monorepo support. The support requires opt-in by both the application and the user. The application must enable this support by setting `allow_git_build_root=True` in the app metadata, and the user must enable support by setting the `<APP_NAME>_EXPERIMENTAL_MONOREPO` environment variable to a truthy value. If both of those are set *and* the app is running inside of a git repository, instead of mounting the current working directory in `/root/project`, the Provider Service mounts the git root in `/root/project` and sets the working directory inside of the provider to the relevant subdirectory of `/root/project`. This way a project that needs access to something outside of the project directory (but still within the same git repository) may access it by setting the `source` of a part to a relative directory walking up the FS tree (e.g. `../`). CRAFT-5272