Add open_dataarray to BackendEntrypoint?

Is your feature request related to a problem?

The BackendEntrypoint class at https://github.com/pydata/xarray/blob/1424972aec3d345a77daea182f6ca0239cb3497d/xarray/backends/common.py#L660-L738 currently has open_dataset and open_datatree methods, but not open_dataarray. Currently, someone implementing a BackendEntrypoint to do xarray.open_dataarray(..., engine="<something>") would have to implement open_dataset, and behind the scenes, this could result in a roundtrip xr.DataArray -> xr.Dataset -> xr.DataArray conversion, which can lead to some attributes/state getting lost along the way (xref https://github.com/pydata/xarray/issues/3268).

Describe the solution you'd like

Enable open_dataarray to be implemented directly, without a roundtrip conversion to/from xr.Dataset

Describe alternatives you've considered

No response

Additional context

https://github.com/GenericMappingTools/pygmt/pull/4008#discussion_r2224256129

Start in xarray/backends/common.py at the BackendEntrypoint class and read the existing open_dataset and open_datatree methods. Trace the xarray.open_dataarray engine path to understand where the backend entry point is selected. Done means a backend can provide open_dataarray directly without the DataArray-to-Dataset-to-DataArray roundtrip, with relevant behavior covered by tests.