Python Guest-side Dependency Setup

# 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`:

https://github.com/influxdata/datafusion-udf-wasm/blob/9e3d58a95f8fad07d9476f34b9dca878dd11c117/guests/python/Justfile#L141-L150

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 to read the dependencies and then call into `pip`, `uv`, or something else within the guest to install said dependencies.

Start with the Python guest setup in guests/python/Justfile, especially lines 141-150, and read the context from #133. Review the inline script metadata specification and compare the possible pip or uv approaches. Done means Python guests retain the default site-packages while being able to declare and install additional dependencies.

Written by the indexing model from the issue text.