Model update in MediaPipe Pose and MediaPipe Holistic

• Better pose landmark accuracy with the updated pose detection and pose landmark model
• Added a lite and a heavy version of the pose landmark model, in addition to the existing full version (also updated and improved)
• Added a model_complexity config option in MediaPipe Pose and MediaPipe Holistic Solution APIs to select across the 3 model versions
• Removed the upper_body_only option in MediaPipe Pose and MediaPipe Holistic, as the standard model now already handles upper-body-only use cases well