REST API: make Knowledge Gaps available in AskPlan to deal with during planning

[JackJackie]

Something to consider is to not only make knowledge gaps available on AskResult, but also on AskPlan. In some scenario's you want to determine knowledge gaps without actually executing the AskKnowledgeInteraction. The admin-ui could, for example, use this to show which knowledge interactions have knowledge gaps, but we do not want to have to execute the AskKnowledgeInteraction before determining the knowledge gaps.

The question is how the REST API can support this, as it does not distinguish between planning and executing. If needed, only the Java API should support this.

Originally posted by @bnouwt in #532 (comment)