Move to extension_metadata() instead of extension_field()?

[kylebarron]

It's not quite accurate to return extension_field() because that implies that you know the name of the field and its nullability. We need to expose both the data type and the extension metadata because arrow-rs doesn't have a DataType::Extension but it might be clearer to expose just those instead of wrapping them in a Field with an empty column name. So that would be:

fn data_type(&self) -> DataType;
/// Extension array name
fn extension_name(&self) -> str;
/// Extension array metadata
fn extension_metadata(&self) -> HashMap<str, str>;

[lewiszlw]

Now extension metadata only contains extension name, is extension_metadata method still needed? If needed, I can add it.

[kylebarron]

This crate hasn't added support yet for extension metadata. We will need to add support at some point for it to keep track of the column's coordinate system information, see https://geoarrow.org/extension-types#extension-metadata

[lewiszlw]

Can you give a review on #224? I'd like to support ewkb after this merged.