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.

[pantierra]

Thanks for your input @mneagul. I adjusted as you suggested. This PR overall has advanced an is in a shape to be reviewed (again).

[ciaransweet]

Should we be using pytest.skip when conditions aren't met? I think we'd want to instead fail tests no?

[pantierra]

Thanks for your review, @ciaransweet! I adjusted the code according to your comments.