Check for conversion deps when using refiners.conversion

[Laurent2916]

small test snippet:

cd /tmp
python3 -m venv .venv
. .venv/bin/activate

pip install refiners@git+https://github.com/finegrain-ai/refiners@pr/check_refiners_conversion_deps
python -c 'import refiners.conversion' # should fail, with message: Some dependencies are missing: ... Please install refiners with the `conversion` extra, e.g. `pip install refiners[conversion]`
pip install refiners[conversion]@git+https://github.com/finegrain-ai/refiners@pr/check_refiners_conversion_deps
python -c 'import refiners.conversion' # should work