[BEAM-3293] Add initial MultiMap side input backing type (#15975)
[BEAM-3293] Add initial MultiMap side input backing type (#15975)


chamikaramj/beam,apache/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,apache/beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,lukecwik/incubator-beam