Support string_dtype, etc. in to_geodataframe

[bnaul]

Way back in #954 @tswast and I were discussing support for pyarrow-backed strings, that was added by @chelsea-lin in #1529 by way of the string_dtype etc arguments.

Would it make sense to add the same thing to the to_geodataframe API? Especially now that pandas is moving towards using the arrow backend by default, I think it would be nice to support these types everywhere.

[tswast]

I think that'd make a lot of sense.

[chalmerlowe]

I will take a look at this and see what we can come up with, unless you are already looking at a PR @bnaul