Planetary computer url signing

[jsignell]

While working on #26 I noticed that there is a kwarg in that library called patch_urls that is used in the docs to sign urls for planetary computer.

Right now xpystac depends on the user to have set up the catalog to sign urls in place and it additionally signs the urls in the reference file if planetary_computer is importable.

It would probably be more general to switch xpystac to use the approach as well and accept a patch_urls kwarg in to_xarray for pystac objects.