Allow users to define the x.y coordinates (similar to set_geometry in geopandas)

[snowman2]

Currently only x,y and latitude,longitude are supported. But, sometime people use lat& lon etc... See: https://gis.stackexchange.com/questions/328128/extracting-data-within-geometry-shape/328320#comment535454_328320

It would be useful to allow users to define it themselves. Either set_geometry or set_xy?
Also, this would mean that the check for x,y coordinates would have to be delayed until users use a function that needs it.

Current workaround:

>>> import xarray
>>> xds = xarray.Dataset(coords={"lon": [1, 2], "lat": [3, 4]})
>>> xds
<xarray.Dataset>
Dimensions:  (lat: 2, lon: 2)
Coordinates:
  * lon      (lon) int64 1 2
  * lat      (lat) int64 3 4
Data variables:
    *empty*
>>> xds.rename({"lon": "longitude", "lat": "latitude"})
<xarray.Dataset>
Dimensions:    (latitude: 2, longitude: 2)
Coordinates:
  * longitude  (longitude) int64 1 2
  * latitude   (latitude) int64 3 4
Data variables:
    *empty*