Google PaLM support

[meal]

Implementation of Google PaLM2 API (Vertex AI version)

For authentication, it uses default credentials, so user must be authenticated through gcloud auth

Missing implementation:

• examples for chat to include them in the request (Examples help the model understand what an appropriate model response looks like. You can write your own example input and output or use the Test section to save a real response as an example. You can also add a prefix which will be appended to every example (for instance, “question” and “answer”)
• tests

[zbigniewsobiecki]

LGTM