feat: Add pubsub notifications

[pantierra]

Related to: EOEPCA/resource-discovery#167

• This PR adds PostgreSQL notification triggers to the pgstac database that automatically send real-time notifications when STAC items are inserted, updated, or deleted. This enables event-driven applications to respond to catalog changes.
• It includes eoapi-notifier middle-ware to listen on these notifications and redirect notification messages as cloudevents to knative (alternatively mqtt can also be used)

[mneagul]

Some of the changes recommended by me. The need for a KSVC is not clear.

[mneagul]

Why do you need a knative service ? Generation of events does not need knative-serving.

[mneagul]

I would change this to something like:

    outputs:
      - type: cloudevents
        config:
          source: /eoapi/pgstac
          event_type: org.eoapi.stac.item
          destination:
             ref:
                 apiVersion: messaging.knative.dev/v1
                 kind: Broker
                 name: my-channel-1
                 namespace: serverless

[mneagul]

The ideea is that when you create a SinkBinding in your helm chart it will result in a fully qualified URL to the Broker/Channel/Service, whatever else is referenced in the ref specification.

Also, besides ref I would also add url which should be mutually exclusive with ref. In case it is set it should be an URL to which you can POST the CloudEvent.