text: Title: [Alerting] Add warnings for usage of pre-defined ID in rule creation API in non-default spaces.
  Body: We currently allow users to pass in a pre-defined ID when creating rules via to the API. Due to breaking changes to make rule SOs share-capable (in which SO IDs will be regenerated), we would like to start logging warnings when this feature is used during rule creation in non-default spaces.
  
  - Add a warning response header when calling the create rule HTTP API with a custom ID in the non-default space
  - Add a warning in our docs about using pre-defined ID with the create rule HTTP API in the non-default space (7.12 to 7.16 docs)
  - Add a server log warning when calling the create rule HTTP API with a custom ID in the non-default space