ENH: Add StringArray.__arrow_array__ for conversion to Arrow
Similar as https://github.com/pandas-dev/pandas/pull/28368 (which did this for IntegerArray)
The failing check is codecov, so ignoring that > The failing check is codecov, so ignoring that

However, we should probably ensure we are testing somewhere pyarrow 0.15 / latest release (from a quick look that doesn't seem to be included on any build). Will open an issue for that.
