[Enhancement]: Extract variables metadata from casedocs namelist files (e.g., `atm_in`)

### Is your feature request related to a problem?

@chengzhuzhang mentioned it would be useful for SimBoard to display the variable metadata (e.g., variable name and frequency) for each case/execution as it enables quick filtering for desired variables.

### Describe the solution you'd like

TBD, consult with team on this feature.

Initial requirements:
1. Add parsers on SimBoard backend for required files
2. Update NERSC ingestion script to utilize parsers

### Describe alternatives you've considered

_No response_

### Additional context

_No response_