Expose rule.consequence.promote.objectIDs string array

On top of the existing `objectID` (singular) string field, a new `objectIDs`
(plural) string array field is now be exposed in the `rule.consequence.promote`
Rule object representation. This lets the user easily promote a full group of
objects without letting some priority conflicts to shift some objects at the end
of the promoted object list.

Locate the Scala representation of the Algolia rule consequence promotion object and compare its existing objectID field with the requested objectIDs field. Confirm how rule objects are modeled and exposed, then verify that the plural string array is available alongside the singular field without changing the existing representation.

Written by the indexing model from the issue text.