Support for rgba8 image encoding.

[Stoiber-dkar]

I have a camera that publishes images in rgba8 image encoding, which is not supported. I got the errors:

• [ERROR] Invalid color image encoding: rgba8
  probably triggered at https://github.com/NVIDIA-ISAAC-ROS/isaac_ros_nvblox/blob/main/nvblox_ros/src/lib/conversions/image_conversions.cpp#L53
• [ERROR] Failed to transform color image.
  probably triggered at https://github.com/NVIDIA-ISAAC-ROS/isaac_ros_nvblox/blob/6c935425605972d5ecf9bea6c3a12006955083b9/nvblox_ros/src/lib/nvblox_node.cpp#L875C5-L875C17

The functions colorImageFromNitrosViewAsync has conversions from rgb8 and bgra8 to rgba8. So i guess there should be support for rgba8 without conversions.