add ob_code column to the target table in the targetdb

eric requested to add a column, ob_code, to indicate an unique target in a proposal to target table in the targetdb. It is useful to identify entries corresponding to a pair of (obj_id, input_catalog_id) recorded in pfsDesign/pfsConfig files and opdb.

Requirements for ob_code is the following.

• string value (varchar)
• unique in a proposal
• a pair of (proposal_id, ob_code) is unique in the target table

I think it is reasonable and won't make any side-effects, so will implement before the December 2022 commissioning run.