Runtime configure nrdg dimension for Ridge file read

### What is the feature/what would you like to discuss?

During review of gravity waves for CAM-SIMA (#426), @nusbaume wrote:

https://github.com/ESCOMP/CAM-SIMA/pull/426#discussion_r2484987089

> Instead of hard-coding this here, I feel like we would get better runtime configurability if we just read this from the associated ridge topography file (e.g. the `nrdg` dimension). I realize we don't have any functionality to specifically grab a dimension from a NetCDF file yet (without directly using PIO calls), so I'd say let's just make an issue for it and then tackle it once CAM-SIMA issue #433 has been implemented.

### Is there anyone in particular you want to be part of this conversation?

@nusbaume

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this