flight_data_from_arrow_batch sends too much data

Arrow arrays can share the same backing store, even if the array is just a "view" of a slice of another array.

Yet, when `flight_data_from_arrow_batch` encodes the arrays into a FlightData, it blindly copies the entire buffer ready to be sent over the wire.

Thus, for example, when DataFusion uses the `arrow::compute::limit` operator to return a few elements of an array, we still end up with a the full (potentially) large array being sent over the wire.

Since encoding the array in a FlightData involves copying the data anyway, perhaps it would be beneficial to take the Array length in consideration and copy only the parts of the buffer that contain actual data.