This PR adds two new decorators, @yields_batches and @yields_elements, which override the default interpretation for the output of DoFn.process and DoFn.process_batch. These decorators are handled via changes to OutputProcessor (now renamed to OutputHandler for clarity), which branches depending on whether batches or elements are expected in the results iterable.

Where possible, common logic in OutputHandler was extracted into private helper methods that are re-used. The .pxd definition for these methods indicates they should be inlined in cython-generated code so they don't affect performance.

Fixes #21656

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