Sorter that will use in memory sorting until the values can't fit into memory and will
then fall back to external sorting.Sorter configured with the given ExternalSorter.Options.SortValues<PrimaryKeyT, SecondaryKeyT, ValueT> PTransform.ExternalSorter.Options contains configuration of the sorter.SortValues<PrimaryKeyT, SecondaryKeyT, ValueT> takes a PCollection<KV<PrimaryKeyT,
Iterable<KV<SecondaryKeyT, ValueT>>>> with elements consisting of a primary key and iterables
over <secondary key, value> pairs, and returns a PCollection<KV<PrimaryKeyT,
Iterable<KV<SecondaryKeyT, ValueT>>> of the same elements but with values sorted by a secondary
key.