REVIEW ONLY: 21755 Release 2: New PRC Streaming

This PR is opened against the 21755-rel-1-new-rpc-streaming branch for review only. Once the first branch is merged I'll either reset the comparison or close and open a new PR for this branch against master. This is because the code contained here is required to go out in a separate relase.

This requires that #108 has been deployed in a release before merging.