pygeoapi plugin for CDM domain model

[isedwards]

We used to have a pygeoapi provider plugin for Climsoft and a separate provider plugin for SURFACE in the old pyopencdms code repository here: https://github.com/opencdms-dev/pyopencdms/tree/52-pygeoapi-provider-plugin-for-surface-cdms/opencdms/pygeoapi

Now we want to replace this with a single provider that is based on the new Climate Data Model. The new code should be in the new pyopencdms repository in /opencdms/pygeoapi/__init__.py to allow people to use from opencdms.pygeoapi import CDMSProvider

Note that this issue should not result in adding any database specific dependendies to pyopencdms but we may need to add an integration test with a database backend - possibly postgresql/TimescalDB with our "compact sample dataset".

Here is the documentation on pygeoapi provider plugins: https://docs.pygeoapi.io/en/stable/plugins.html#example-custom-pygeoapi-vector-data-provider

[isedwards]

We should implement a test similar to pygeoapi's test_postgresql_provider.py