Add support for Databricks-hosted model in Draft Validation

[sansbacon]

Proposal

chatlas supports OpenAI compatible model providers (such as Databricks) but this is not an option for the DraftValidation class. The environment variable for OPENAI_BASE_URL is not used when calling chatlas.

chat = ChatOpenAI( # pragma: no cover
model=model_name,
system_prompt="You are a terse assistant and a Python expert.",
api_key=self.api_key,
kwargs={"http_client": http_client},
)

Since ChatOpenAI supports a base_url argument this should be a simple feature to add.