[ML] Refactor unified schema classes

[jonathan-buttner]

Some of the unified schema classes are organized around openai directly. We should make the class names and locations more generic.

These classes are used by different services and should be renamed and moved:

• https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/openai/OpenAiUnifiedStreamingProcessor.java
• https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/response/openai/OpenAiChatCompletionResponseEntity.java

[elasticsearchmachine]

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