Add iio_backend_read_scale() and iio_backend_read_offset() services
to read channel scale and offset from an IIO backbend device.

Also add a read_raw callback which replicates the read_raw callback of
the IIO framework, and is intended to request miscellaneous channel
attributes from the backend device.
Both scale and offset helpers use this callback.
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240730084640.1307938-2-olivier.moysan@foss.st.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>