fix(cameras): change the key from id to serial_number

[sotanakamura]

What this does

This PR changes the the key from id to serial_number. The key "serial_number" is used in _find_serial_number_from_name

This also includes the fix for mypy type checks of instance variables.

fix #2493

How it was tested

Tested with a real hardware.

How to checkout & try? (for the reviewer)

Connect one RealSense and run the following script:

from lerobot.cameras.realsense import RealSenseCamera, RealSenseCameraConfig

realsense = RealSenseCamera(RealSenseCameraConfig("Intel RealSense D415"))
realsense.connect()
realsense.disconnect()