allow arrow input in accessor

[martindurant]

Currently we require awkward input for the .ak accessor to appear on a series. This is fine, since .astype("awkward") works. But do we want the accessor to also allow string[pyarrow] or arrow type inputs?

[douglasdavis]

I think it makes sense, based on previous conversations I think we want to make it as easy as possible to use awkward as a compute engine on pyarrow types