Support loading records from target tables

[Bob Vawter]

We have the apply package for modifying a target table, but there's no way to load a row into a mutation. This would be a nice-to-have feature in the userscript, as it would allow the TargetTX interface to be able to load a value by PK. This enables a soft-delete use case that doesn't require the changefeed to be created with the diff option.