Add a HST REST API for adding a component to a container at a particular position

Following up on CHANNELMGR-2255 we are now forced to do 2 calls, one to add a component to a container, then one to move the component inside the container. This complicates the logic for rendering components in the overlay in the frontend code.

Currently we choose the easy route and rerender the container twice (once after each call). Aside from being suboptimal in normal situations this could turn out to be a real performance/UX problem for SPA's that rerender the whole page, or for complex containers with lots of components.

Preferably we would be able to add some index to the addComponent call to directly add the component on a specified position in the container.