Add update_on option to batch_update

[frodsan]

I noticed the following pattern in our codebase:

Model.batch_update_statements(
  to_update,
  update_on: %I[foreign_key_id],
).each { Model.connection.execute(_1) }

This adds an update_on keyword argument to batch_update, allowing callers to specify which columns to use for matching records instead of always defaulting to the primary key. I'd prefer to write the previous code like:

Model.batch_update(to_update, update_on: %I[foreign_key_id], columns: %I[...])

[quentindemetz]

Thank you!