directly vendor schema files

[PietroPasotti]

Issue

Pypi packages apparently cannot depend on git+ dependencies.
https://github.com/canonical/cos-lib/actions/runs/12252877996/job/34180522427

charm-relation-interfaces is not on pypi (yet)

Solution

Instead of installing 'interfaces' from git, download them in this repository tree for the time being.
This is a short term workaround, not a solution.
If charm-relation-interfaces is not to be on pypi, a better approach would be to pull the schemas automatically from CI every time there's a merge on CRI main.

[NucciTheBoss]

@PietroPasotti what was the reason for adding cryptography as a dependency in pyproject.toml? As of the latest cosl release, every charm that pulls in cosl will need to include a Rust toolchain during the packing process to compile cryptography, or add an entry for cryptography in charm-binary-python-packages.