Add GET endpoint to retrieve the elements in

Add GET endpoint to retrieve the elements in the model

This method would facilitate querying the model data from external applications in a JSON format, structured like the element properties panel of the iTwin viewer.

The method should allow filtering by element properties, spatial location and return all the element properties (data).

It would be useful to use this api to retrieve geometric abstractions of the elements, such as their insertion point and their bounding box/cube coordinates.