more refactoring on chat and embed model support

* differentiate between input and output tokens
* make use of much larger input contexts

stricter interface inheritance

This ensures we have the apropriate methods when a class does not
inherit from AbstractModel.

Use interfaces for Chat and Embedding classes

This way it's easier to have a base OpenAI class. This also moves much
of the statistics and http handling into the base class making model
implementati

Use interfaces for Chat and Embedding classes

This way it's easier to have a base OpenAI class. This also moves much
of the statistics and http handling into the base class making model
implementations even leaner

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