Make sure that PostgreSQLProvider.get_fields returns valid json schem…

[totycro]

…a types

Overview

All relevant discussion should be in the issue here: #1091

This PR makes sure that the postgres provider only provides valid types, however no validation for all get_fields methods has been implemented so far.

Related Issue / Discussion

#1091

Additional Information

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