[Feature] Support and validate Job notification callback URL

[fmigneault]

Description

Validate the passing of a subscriber-URL in an execute request.

• https://github.com/opengeospatial/ogcapi-processes/blob/master/openapi/schemas/processes-core/execute.yaml
• https://github.com/opengeospatial/ogcapi-processes/blob/master/core/abstract_tests/callback/ATS_process-execute-callback.adoc#L8

relates to #53

Weaver supports notification_email from job execution, but could easily support the equivalent operation with the subscriber schema. https://github.com/opengeospatial/ogcapi-processes/blob/master/openapi/schemas/processes-core/subscriber.yaml
The corresponding job status requests should be set to relevant callback URLs the subscriber defined.

Related

• https://github.com/opengeospatial/PubSub-Whitepaper
  DRAFT report: https://github.com/opengeospatial/PubSub-Whitepaper/blob/master/20-046.pdf
• https://github.com/opengeospatial/pubsub