- [x] Added the fields cover and `start_timestamp` to the class `Video`, containing a message-specific cover and a start timestamp for the video.

- [x] Added the parameters cover and start_timestamp to the method sendVideo,
allowing bots to specify a cover and a start timestamp for the videos they send.

- [x] Added the fields cover and start_timestamp to the classes
InputMediaVideo and InputPaidMediaVideo, allowing bots to edit video
cover and start timestamp and specify them for videos in albums and paid media.

- [x] Added the parameter video_start_timestamp to the methods
forwardMessage and copyMessage, allowing bots to change the start
timestamp for forwarded and copied videos.