Specs
primary_key!(Ecto.Repo.t(), table_name_t()) :: {:primary, [String.t()]} | no_return()
Returns the primary key/ID for a table. It may be a single field that is a HASH, OR it may be the dynamoDB {HASH, SORT} type of index. we return {:primary, [index]} in a format described in normalise_dynamo_index!