Add basic Gen3 butler support to obs_sdss

Add an Instrument class and PipelineTask config overrides sufficient to ingest SDSS data into a Gen3 butler repository.

This may uncover issues in astro_metadata_translator for SDSS, because SDSS has unusual (due to drift-scan observing) identifiers that may be hard to map well to exposures and visits, and this will be the first time we've really exercised them.