(fix): allow all extension array data types in pandas adapters

[ilan-gold]

This probably needs some work, but since pandas datetime handling often goes through extension arrays, I think these two issues are completely linked

• Closes Failure in pandas TestDataFrameToXArray.test_to_xarray_index_types pydata/xarray#9661
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

cc: @shoyer @kmuehlbauer

[ilan-gold]

It's tough to do this without CI - I will push a few of the sorts of things re: datetimes that will change so you get a sense, but generally, things are even cleaner (more datetimes preserved).

[ilan-gold]

Not sure we want to keep allowing pandas in-memory date time stuff + dask: 59b03f2 maybe should start blanket converting extension arrays before dask?

[kmuehlbauer]

@ilan-gold Tried to activate CI, but seems it doesn't work. I'm back to desk next week, won't have time to check this now.

[shoyer]

Can you open this up as a pull request against pydata/xarray?