This is a continuation of the work started in #17429 and #17574 and extends the existing registration logic to allow registration of combiners.

Design doc: https://docs.google.com/document/d/1MLZLUhb-Sn7AJ666yn3TnYgMKE6wiXIBs7cjB_20hMA/edit?usp=sharing

After this, the big remaining chunks of work are:

1. Perf testing
2. Updates to the docs/our examples

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