Python: metkitlib integration

[tmi]

1/ Moves pymetkit into python/pymetkit, for standard organization. However, pyproject.toml remains rooted until ci is adapted
2/ Adds a dependency of metkitlib to pymetkit, so that the libmetkit.so is brought in automatically

I've tested as follows in a clean venv:

uv pip install --prerelease=allow -e .
python -c 'import pymetkit._version; print(pymetkit._version.__version__)' # simple test that it can be imported
pytest ./python/pymetkit/tests

and all looked good

Note that the --prerelease=allow param is needed until all of eccodes, eckit and metkit are released (in the sense of develop->main merge) and then wheels published (in the sense of running corresponding github action from main)