Butler access to exposure metadata objects

FYI DM-9060 Add metadata access to get wcs, visitInfo, and calib from a calexp dataset is checked in, in repositories that use a mapper that inherits from CameraMapper you can now access exposure metadata objects wcs, calib, and visitInfo by adding an underscore + the postfix to the exposure dataset type name, for example calexp_wcs, calexp_calib, and calexp_visitInfo.