[BC-Breaking] Change streaming decoding to return Iterator instead

[moto-meta]

Summary:
Context:

In #1264, the C++ Generator has been fixed and now we can bind the Generator object to Python, which allows to create Python Iterator object out of C++ Generator.

This makes the implementation of streaming decoding simpler and more aligned with how the underlying FFmpeg decoders work.

With #1268 and #1271, we adopted to NVDEC decoder, which in turn allowed to batch allocate the memory for decoded buffers, improving the performance

This commit applies the similar approach to software decoder. Although the memory allocation for the decoded frame is controlled by FFmpeg, so we don't have the same benefit as the case of NVDEC.
However, the resulting change in the user-facing interface "Optional[Frame]" -> "Iterator[Frame]" makes the handling of decoded frames less awkward as it naturally handles the case where the feeding the next packets or flushing will generate multiple batches of frames.

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Differential Revision: D90396373

[meta-codesync]

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