add logprobs to generation_info (metadata)

[epistoteles]

This PR adds logprobs to the response_metadata of the chat completion request. If there are no logprobs available the logprobs will be None. top_logprobs will be included in the metadata if they are requested.

Merging this PR will enable you to turn this logprobs ❌ into a ✅: https://python.langchain.com/docs/integrations/chat/databricks/

Implements #69

[epistoteles]

Note: It would be nice to include somewhere in the docs that this is how you have to request logprobs:

chat_model = ChatDatabricks(
    endpoint="gpt-4o-mini",
    temperature=0,
    max_tokens=1,
    extra_params={
        "logprobs": True,
        "top_logprobs": 20
    }
)

just setting

chat_model = ChatDatabricks(
    endpoint="gpt-4o-mini",
    temperature=0,
    max_tokens=1,
    logprobs: True,
    top_logprobs: 20
)

will not throw an error but also not work.