add CameraRos to rmf_demos_assets, add humans and camera to hotel world

[PranayShirodkar]

Background

These changes are part of the rmf_obstacle feature. This PR is to add obstacle detectors (cameras) and obstacles (humans) into the rmf hotel demo. This PR is related to rmf_human_detector PR.

Changes

• add CameraRos model to rmf_demos_assets, CameraRos model contains plugins to publish camera image topic and camera pose topic
• add CameraRos and human models to hotel.building.yaml

Note: Once rmf_sandbox is ready, we should not need a CameraRos model. A user will be able to configure camera params in rmf_sandbox and rmf_sandbox will generate the camera's model.sdf and add it to the world sdf.