Be able to use Codey models on Vertex AI (#6354)
Added the functionality to leverage 3 new Codey models from Vertex AI:
- code-bison - Code generation using the existing LLM integration
- code-gecko - Code completion using the existing LLM integration
- codechat-bison - Code chat using the existing chat_model integration
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Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>
