Add VertexAI, simplify provider creation.

[rasmi]

Depends on #1045

• Add VertexAI as a model provider (either API key-based ("express") or service account-based auth).
• Clean up provider creation (get credentials function is not necessary, just pass the full credentials config).

[cjqian]

Cool! Can you make sure this is mentioned in the appropriate places in the documentation?

[rasmi]

@cjqian Done!