Databricks LLMs service endpoints implementation

[t0mpl]

Databricks hosts some LLMs and expose API for them.

NB: with the multiplication of model providers, maybe the langchaingo structs formatter should be separate from the client. We will duplicate some code otherwise.

PR Checklist

• Read the Contributing documentation.
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[FluffyKebab]

Looks very good. Can you add doc.go files in the new packages you made? We should also have an example of how to use this LLM, but that can be done in a separate PR

[t0mpl]

Hey FluffyKebab, thanks for the review, made the changes you've asked