Add BatteryStates msg

[YaraShahin]

Description

This PR introduces a new message definition: BatteryStates.

• BatteryStates contains an array of sensor_msgs/msg/BatteryState messages.
• The purpose is to support broadcasting the status of multiple batteries from a single source.
• This message will be used by the battery_state_broadcaster in ros2_controllers: ros-controls/ros2_controllers#1888.

Is this user-facing behavior change?

No

Did you use Generative AI?

No

Additional Information

This PR is being opened following discussion in ros-controls/control_msgs#250. The BatteryStates message was initially proposed for control_msgs, but @saikishor recommended placing it in sensor_msgs instead, since:

• It's built on top of sensor_msgs/BatteryState.
• It represents sensor data rather than control-specific info.
• Other projects beyond ros2_controllers may have similar use cases for representing multiple battery states.