default item transactions to GeoJSON payloads

[tomkralidis]

Overview

Default to GeoJSON payloads for items transactions in OpenAPI.

Related Issue / Discussion

This is also required on SwaggerUI given default content type of application/json will kick into CQL mode instead.

Additional Information

Will be updated once opengeospatial/ogcapi-features#771 is properly addressed.

Contributions and Licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution.
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[tomkralidis]

CI failures are unrelated per #1165