Add Cerebras Inference LLM provider

[ssojoodi]

Description

Adding support for Cerebras Inference as a new LLM provider.

Checklist

• [x] The base branch of this PR is dev, rather than main
• [x] The relevant docs, if any, have been updated or created

Screenshots

Testing

Go to Cerebras to get an API key. https://cloud.cerebras.ai/ Once Continue extension is added in VS Code (or web) you can select Cerebras as a model provider and Llama3.1 8B or 70B as the model. Then enter API key and add. This enables the fast inference provided by Cerebras.