Expose `coord_mode` in `.pad()`

Is your feature request related to a problem?

I am experiencing some issues with the way coordinates are padded. Currently the coordinates are padded according to the mode parameter passed to pad() here. For my particular issue I want to be able to have different modes for the data, but effectively pin the mode that is used to pad the coordinate values.

A simple example:

import xarray as xr
import numpy as np

da = xr.DataArray([1,2,3], dims=['x'], coords={'x':[4,5,6]})
da

If I pad with constant

da.pad(x=(0,1), mode='constant')

I get this:

which is the coordinate padding I need.

But if I pad with wrap:

da.pad(x=(0,1), mode='wrap')

which gives me repeated coordinate values

Describe the solution you'd like

I would like to be able to do something like this:

da.pad(x=(0,1), mode='wrap', coord_pad_mode='constant')

and get something like this

Since #3596 internally already defines coord_pad_mode it should be easy enough to expose this to the user?

Possibly related (but I think not incompatible with the changes propsed here?): #3868

Happy to work with @TomNicholas on a PR, but wanted to get some feedback/comments first.

cc @dcherian @mark-boer who worked on this code.

Describe alternatives you've considered

No response

Additional context

No response

Start in xarray/core/dataset.py around line 7342 and trace how pad() passes its mode to coordinate padding; inspect the prior coord_pad_mode work referenced in #3596. Add the public option so the example can use wrap for data and constant for coordinates, then verify that the resulting coordinates match the requested behavior.