Python Guest-side Dependency Setup

[crepererum]

What

Allow Python guests to install new dependencies.

Why

Currently we create a fixed root file system for the Python guests that includs site-packages:

datafusion-udf-wasm/guests/python/Justfile

Lines 141 to 150 in 9e3d58a

	"{{PYTHON_TMP_VENV}}"/bin/pip install \
	--disable-pip-version-check \
	--no-deps \
	--ignore-installed \
	--implementation=py \
	--isolated \
	--only-binary=:all: \
	--python-version={{PYTHON_VERSION_FULL}} \
	--requirement=requirements.txt \
	--target="{{PYTHON_SITE_PACKAGES}}"

This is fine by default and I think we should continue doing so, but we should also allow Python guests to extend that setup.

How

Wait for #133. Then way may use some metadata a la https://packaging.python.org/en/latest/specifications/inline-script-metadata/#inline-script-metadata to read the dependencies and then call into pip, uv, or something else within the guest to install said dependencies.