Add new kwarg `drop_existing` to `set_xindex`

[ianhi]

Is your feature request related to a problem?

Currently if you want to set an xindex on a coord that already has an index you have to remember the slightly differently named drop_indexes and to write out all the coord names twice.

da = xr.DataArray([0,1,2,3], dims=('x'), coords={'x': np.linspace(0,1,4)})

# need to repeat all the variables
da.drop_indexes('x').set_xindex('x', CustomIndex)

# ValueError: those coordinates already have an index: {'x'}
da.set_xindex('x', CustomIndex)

Describe the solution you'd like

Give set_xindex a drop_existing argument (default to False) that automatically drops whatever is necessary to complete the index setting the user requested.

Describe alternatives you've considered

No response

Additional context

No response

[max-sixty]

should set_index be an upsert?