Include Source Grid/Mesh File Attribute in CESM Output

It would be very helpful if CESM outputs retained a reference to their source grid or mesh files. Currently, there is a `topography_file` attribute that references a file, but nothing specifically references the source grid.

For instance, one of the projects I work on ([UXarray](https://github.com/UXARRAY/uxarray)) relies on accessing the original grid to correctly load and analysis CESM data in its native format.

### Example

A file I'm currently working with contains the following attribute:
```
topography_file :
/glade/campaign/cesm/cesmdata/cseg/inputdata/atm/cam/topo/USGS-gtopo30_ne120np4_16xdel2-PFC-consistentSGH.nc
```

However, identifying the source grid file took additional effort. The source grid file is located here:

```
/glade/p/cesmdata/cseg/inputdata/share/meshes/ne120np4_ESMFmesh_cdf5_c20211018.nc
```

Having a dedicated attribute like the following would significantly streamline workflows:

```
mesh_file :
/glade/p/cesmdata/cseg/inputdata/share/meshes/ne120np4_ESMFmesh_cdf5_c20211018.nc
```

Thank you!