[ML] Inference API unified stream processing duplication

[jonathan-buttner]

This class OpenAiUnifiedStreamingProcessor and this class OpenAiStreamingProcessor are very similar.

The main difference is the way the result that is returned. We should refactor them such that most of the work is done by OpenAiUnifiedStreamingProcessor.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[wangzhenhui1992]

emmm