Ship function documentation, improve remote st_read calls

[Maxxen]

We now include the documentation for spatial function in the extension itself, so examples and descriptions will show up when you call

SELECT * FROM duckdb_functions()

We use then use this in a python script that constructs a query selecting all spatial functions and dump them to a markdown file available in docs/functions.md. There's still some stuff left to do:

• COPY functions are not part of the output and are missing docs
• Far from every function has an example
• Parameter names need to be documented manually and haven't been done yet

I've also done some tweaks around with st_read and remote calls should now be much faster as we don't force compute the feature count if its expensive and also make use of gdal cached file handles when querying remotely.

I've also added a st_hilbert scalar function that is completely untested but I want to use in the future to prototype some basic spatial index stuff