feat(engine): manage state of not configured retrieval (#408)

Description

Manage state of not configured retrieval:

• add a new status (pending) for the retrieval service,
  • it signals that the engine is managed by Marketplace and has not been configured yet
• seek out the pending.retrieval file as a specific marker to detect the pending state
• removes pending.retrieval file after running POST /admin/config
  • if the file is exist
  • if the config is valid
• not to start the data refresh process until the managed DLE instance is configured

Related issue

#408 (closed)

Examples

Checklist

• MR description has been reviewed
• MR changes are functionally tested
• MR does NOT have text changes OR there are text changes and they have been reviewed
• MR does NOT have API/CLI changes OR there are API/CLI changes and they have been reviewed
• MR does NOT have UI changes OR there are UI changes and they have been reviewed

Closes #408 (closed)