Split NVCF generator into completion chat

[leondz]

NVCF models support at least two interfaces - Completion and Chat styles. They also sometimes require custom headers. This PR splits the NVCF class into two, one for each interface, providing separate methods for building the payload and parsing the output under each interface style. It also adds code to allow external configuration of the payload fields that the NVCF endpoint is queried with.

[leondz]

ps. i know, i know, sorry about the _config indexing jeffrey

[leondz]

Stray thought as I looked at this, are these services really just OpenAICompatible like NIM? ( This is likely a futures effort thought. )

One would hope so! I like where this is going.