Support google gemini

[joy13975]

This change allows one to use langchain-google-genai and simply

generator = TestsetGenerator.with_google(...)

just like one would with with_openai() and get on with life.
Initial tests show no errors in embedding and testset generation. Prompt adaptations work on condition of #676 being merged first.

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CI/CD check fail: langchain-google-genai requires Python >=3.9, <4.0

[jjmachan]

what do you think of making this an optional dependency?
something like ragas[google].

I'm waiting for our approval to use GCP so will test these after that

[joy13975]

I actually think #670 could be a better way of dealing with different vendors.
However, one thing that still needs to be fixed is temperature arg being a bit of a mess (having and arg but not actually using it) + being unnecessarily (?) passed into the Langchain LLM it sometimes causes errors if the Langchain LLM isn't expecting it as an arg such as in #656

[jjmachan]

hey @joy13975 I agree that #670 is turning out to be the better implementation, so we'll merge that in.
as for #656 yeah that needs work and will try to get it sorted