allow merging datasets where a variable might be a coordinate variable only in a subset of datasets

Is your feature request related to a problem?

Here are two datasets, in one a is a data_var, in the other a is a coordinate variable. The following fails

import  xarray as xr

ds1 = xr.Dataset({"a": ('x', [1, 2, 3])})
ds2 = ds1.set_coords("a")
ds2.update(ds1)

with

    649 ambiguous_coords = coord_names.intersection(noncoord_names)
    650 if ambiguous_coords:
--> 651     raise MergeError(
    652         "unable to determine if these variables should be "
    653         f"coordinates or not in the merged result: {ambiguous_coords}"
    654     )
    656 attrs = merge_attrs(
    657     [var.attrs for var in coerced if isinstance(var, (Dataset, DataArray))],
    658     combine_attrs,
    659 )
    661 return _MergeResult(variables, coord_names, dims, out_indexes, attrs)

MergeError: unable to determine if these variables should be coordinates or not in the merged result: {'a'}

Describe the solution you'd like

I think we should replace this error with a warning and arbitrarily choose to either convert a to a coordinate variable or a data variable.

Describe alternatives you've considered

No response

Additional context

No response

Reproduce the failure with the two datasets in the issue and start from the merge path that raises the shown MergeError. Determine how the merged result should classify the conflicting variable, then add coverage for the dataset update case and verify that it no longer raises the ambiguity error.