Implement OGC API Features Part 4 transactions for the postgres provider

[totycro]

Overview

This PR implements create(), update() and delete() in the postgres provider. The behavior is analogous to implementations in other providers and as defined by the standard.

The implementation is pretty basic, i can add more advanced logic if necessary (e.g. inputs which are not in WGS 84).

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

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]

PR looks good, thanks @totycro !

Please mark transactions as supported in PostgreSQL in the providers table.