Consider clearing extension type metadata when modifying Field::data_type

I missed this when adding extension type methods, but perhaps we should check if a field has extension type information when updating the data type, and if it does, drop the extension type (because we can't validate that the new data type is supported).

_Originally posted by @mbrobbel in https://github.com/apache/arrow-rs/pull/7497#discussion_r2086981702_

Start from the Field::data_type update path and the extension type methods referenced in the issue. Check how extension type information is stored and cleared when the data type changes; done means an updated data type cannot retain unsupported extension metadata.

Written by the indexing model from the issue text.