Add support for custom Deepstack models

[neilenns]

Fixes #416

Description of changes

• Add an optional customEndpoint property to the trigger configuration
• Update the schema to require either customEndpoint or watchObjects but not both
• Use the customEndpoint property when calling Deepstack if it is specified, and skip checking watchObjects when it is included

• Checklist

If your change touches anything under src or the README.md file these items must be done:

• [X] User-facing change description added to unreleased section of CHANGELOG.md