Implemented a GetAllModels function for the RobotImporter

The GetAllModels function is able to gather all the models from SDF document, an SDF world or models nested within another SDF model.

In addition, overloads of the RobotImporter `VisitModels` function have been added to support visting models of an SDF world or SDF model directly for the user.

Signed-off-by: lumberyard-employee-dm <[email protected]>

Author: lemonade-dm