Add Coordinates.assign() method

[benbovy]

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

This is consistent with the Dataset and DataArray assign methods (now that Coordinates is also exposed as public API).

This allows writing:

midx = pd.MultiIndex.from_arrays([["a", "a", "b", "b"], [0, 1, 0, 1]])
midx_coords = xr.Coordinates.from_pandas_multiindex(midx, "x")

ds = xr.Dataset(coords=midx_coords.assign(y=[1, 2]))

which is quite common (at least in the tests) and a bit nicer than

ds = xr.Dataset(coords=midx_coords.merge({"y": [1, 2]}).coords)