YOLOv8 instance segmentation

[martin-gorner]

Add instance segmentation to the existing YOLOv8 object detection model. This can be a Keras.io example first and get finalized as a KerasCV contribution in a second step. Numerical validation agains a reference model is a necessary. Reference implementation of the instance segmentation head here: https://github.com/ultralytics/ultralytics/blob/6852b76611744bec5cb36ced3b8f64250e6c0435/ultralytics/cfg/models/v8/yolov8-seg.yaml#L6