feat: add to_geo() method

[cboettig]

Describe the solution you'd like

duckdb spatial extension provides the ability to both read a wide variety of spatial formats (geojson, flatgeobuf, etc) and write data out into these formats (duckdb docs).

ibis gives us read_geo() to read all those formats, but no to_geo() method to write back out. It would be awesome to have a to_geo() method!

Being able to write out to these formats is a great way to make sure ibis geospatial workflows can easily interoperate in workflows with other spatial software or with other users that require these formats.

What version of ibis are you running?

9.5.0

What backend(s) are you using, if any?

duckdb

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