Add support for GeminiPro

[kokuyouwind]

I want to use Google's gemini-pro model.
This is included in Vertex, but while other models such as bison require only prompt as a parameter(reference), gemini-pro requires a chat-style parameter with a role(reference).

LangChain in Python implements this model separately from ChatVertexAI under the name ChatGoogleGenerativeAI. It would be desirable to do the same in Ruby.

[kokuyouwind]

According to googleapis/google-api-ruby-client#17071, there is currently a bug in using the gemini pro model with Google::Apis::AiplatformV1 (I reproduced it on my environment).
There are approaches to use the REST API directly (like https://github.com/gbaptista/gemini-ai), but it would be better to wait until it is available from the official client.

[andreibondarev]

@kokuyouwind If you'd like to work on a PR I would welcome it with open hands! 😄