Mutation to add/remove to/from column value

Based on this request:

Don’t have separate mutations, rather expand the mutation objects for these types where add/remove are relevant like the following:

{
  dropdown: { add_labels: ["label", "label3"], remove_labels: ["label2"] },
  board_relation: { add_item_ids: ["123123"], remove_item_ids: ["543543"] }
}

The functionality should be obvious.

The add/remove would be unions with the existing labels (or other values) or if a labels key is specified, unions with that rather than existing value. Alternatively it could be an error to mix add/remove and the standard labels.

This way we can do other mutations in the same request.

CC: @anon29275264