Optionally include encoding in Dataset to_dict

[jhamman]

Is your feature request related to a problem?

When using Xarray's to_dict methods to record a Dataset's schema, it would be useful to (optionally) include encoding in the output.

Describe the solution you'd like

The feature request may be resolved by simply adding an encoding keyword argument. This may look like this:

ds = xr.Dataset(...)
ds.to_dict(data=False, encoding=True)

Describe alternatives you've considered

It is currently possible to manually extract encoding attributes but this is a less desirable solution.

xref: pangeo-forge/pangeo-forge-recipes#256