Move pygeoapi CDMSProvider plugin to this separate repo

[isedwards]

• Move pygeoapi provider to this separate pygeoapi-cdms-plugin repo and remove associated code, including dependencies, from pyopencdms
• If possible, replace DTOs by upgrading our existing data classes to pydantic data classes
• Current plugin is using SQLAlchemy db.session to query our models.Observation domain model. It's probably okay for now but we should raise an issue to ensure future querying is via the domain model methods - which in turn may choose to use SQLAlchemy depending on the data store being used
• Use Black for code formatting