Add support for Writer models

[samjulien]

Why are these changes needed?

Adding support for LLMs by Writer (for now, our latest Palmyra-X-004 model). We'd like for Palmyra-X-004 to be available on the Chatbot Arena.

Let me know what else you may need from me, e.g. API key.

Thank you!

Checks

• [X] I've run format.sh to lint the changes in this PR.
• [X] I've included any doc changes needed.
• [X] I've made sure the relevant tests are passing (if applicable).