Use json format instead of xml for describe_feature

[This is probably unnecessarily convoluted](https://github.com/bcgov/bcdata/blob/master/R/describe-feature.R#L78), and there is an `outputFormat=application/json` option that is probably easier to parse than the xml and has more info. There are some differences in the fields however (eg. https://openmaps.gov.bc.ca/geo/pub/wfs?SERVICE=WFS&VERSION=2.0.0&REQUEST=DescribeFeatureType&outputFormat=application/json&typeNames=WHSE_ENVIRONMENTAL_MONITORING.EMS_MONITORING_LOCN_GROUPS_SVW) - notably the `GEOEMETRY` field is `gml:Geometry`, whereas the xml output `type="gml:GeometryPropertyType"`. The json has a "local type" which the xml does not.