id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
1880544087,I_kwDOAMm_X85wFtNX,8144,Add support for netcdf4 enum,16700639,closed,0,,,10,2023-09-04T15:51:45Z,2024-01-17T07:19:33Z,2024-01-17T07:19:33Z,CONTRIBUTOR,,,,"### Is your feature request related to a problem?

When a netcdf file contains [netcdf4 enums ](https://unidata.github.io/netcdf4-python/#enum-data-type), xarray ignores the underlying enum type.
The association between the values of the variable and their actual meaning is then lost.

MRE:
```py
import netCDF4 as nc
import xarray as xr

# -- Create dataset with an enum using the netcdf4 lib
ds = nc.Dataset(""mre.nc"", ""w"", format=""NETCDF4"")   
cloud_type_enum = ds.createEnumType(int,""cloud_type"",{""clear"":0, ""cloudy"":1})
print(ds.enumtypes)
# {'cloud_type': <class 'netCDF4._netCDF4.EnumType'>: name = 'cloud_type', numpy dtype = int64, fields/values ={'clear': 0, 'cloudy': 1}} 
ds.createVariable(""cloud"", cloud_type_enum)
ds[""cloud""][0] = 1
ds.close()

# -- Open dataset with xarray
xr_ds = xr.open_dataset(""./mre.nc"")
print(xr_ds.cloud)
# <xarray.DataArray 'cloud' ()> \n [1 values with dtype=int64]   
# --> We get no metadata about the cloud_type enum that we created above 
xr.ds.to_netcdf(""mre_xr.nc"")

# -- Open xarray outputted dataset with netCDF4 lib
print(nc.Dataset(""mre_xr.nc"", ""r"", format=""NETCDF4"").enumtypes())
# {}
# --> Empty dictionary: the enum we created is lost
```

If you know CF, enums could replace replace `flag_meanings` and `flag_values`, see [CF](http://cfconventions.org/cf-conventions/cf-conventions.html#flags)
Enums are [not yet ](https://github.com/cf-convention/discuss/issues/238) part of CF though.

### Describe the solution you'd like

As far as I understand, to describe the enum we only need a dictionary that map numbers (enum key) to string (enum value) and a way to reference this dictionary in variables that are ""typed"" to this enum.
Bear in mind that the dtype of the variable would still be a number, the enum type would be a secondary metadata.

### Describe alternatives you've considered

Most people that produce data could get away with using flag_meanings and flag_values to describe their data in a way which is both CF proof and properly managed by xarray.
For me, the only workaround at the moment is to use the netCDF4 library directly.


### Additional context

```py
nc.__version__
# 1.6.2

xr.__version__
# 2023.2.0
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