2D mode like robot_localization

[adamsj-ros]

Is your feature request related to a problem? Please describe.
robot_localization provides a 2D mode of operation which helps when using Nav2 (ROS2). For better or worse, Nav2 works in 3D (meaning poses are 3D) but many operations imply the use of 2D points. A 2D mode supplied by the robot_localization package make this easier to work with.

Describe the solution you'd like
I was hoping something like this would be implemented.

Describe alternatives you've considered
I could republish the topic where I set the two d mode outside of the package but that wouldn't be as performant.

Additional context
None.