feat: [livestream] added support for slate events which allow users to create and insert a slate into a live stream to replace the main live stream content#9665

lqiu96 merged 2 commits intomaingoogleapis/google-cloud-java:mainfrom

owl-bot-copy-java-video-live-streamgoogleapis/google-cloud-java:owl-bot-copy-java-video-live-streamCopy head branch name to clipboard