fix: keep type on flat item

[gadomski]

Though the spec recommends dropping it, I'd like to keep it for now:

• There's already at least one other probably-identical string field in the file (stac_version) so if we're trying to optimize on size we could play with metadata more to cut more columns
• Naïve geoparquet readers are more likely to be able to reconstruct valid STAC items if all the required fields are present ... without type, you really have to know what you're doing
• In our case, when we search stac-geoparquet we don't assume that we're returning valid STAC items, because the fields extension can be used to strip away required fields. Because of that, a direct conversion from stac-geoparquet to JSON will not "rehydrate" the type field. We could add logic to search to do this "rehydration", but it seemed simpler and not super-high-consequence to just keep the column

cc @hrodmn

[fwfichtner]

Thanks for adding this @gadomski. We recently changed from stac-geoparquet to create parquet to be used in stac-fastapi-geoparquet to rustac-py and with type missing the API is not valid anymore and cannot be searched.

[gadomski]

FYSA a PR to fix over at stac-geoparquet is here: stac-utils/stac-geoparquet#105, though it's not merged yet.