Add angle-based servo control example

[dbaldwin]

Summary

• Adds examples/example_servo_angle.py, a small rclpy client that drives a PCA9685 channel by angle (0-180°) via /dexi/servo_control.
• Uses the service's natural angle interface; pulse-width range falls back to the node's defaults (500-2500 µs) unless --min-pw / --max-pw are passed.
• CLI: python3 example_servo_angle.py <channel> <angle> [--min-pw US] [--max-pw US] (defaults: channel 0, angle 90).

Test plan

• Launch the servo node: ros2 launch dexi_cpp servo_controller.launch.py
• Run python3 examples/example_servo_angle.py 0 90 and confirm the servo on channel 0 centers
• Sweep a channel through 0 / 90 / 180 and confirm endpoints + center
• Run with --min-pw 1000 --max-pw 2000 and verify the narrower range is honored
• Confirm out-of-range channel (e.g. 16) or angle (e.g. 200) returns success=false