feat: add external API for DeepSeek LLM

[keturn]

Summary

Adds a node to use the external DeepSeek API provider for LLM tasks.

Can be used as an alternative to the Text LLM node.

Proposing this for inclusion instead of a third-party node so we can build on it in the Expand Prompt UI.

Related Issues / Discussions

🗨️ developer-forums / external LLM providers

QA Instructions

• Obtain API key from https://platform.deepseek.com/api_keys
• Add it as external_deepseek_api_key to invokeai.yaml or api_keys.yaml
• Run a workflow with the DeepSeek Text LLM node. (No other nodes required.)

Merge Plan

No current complications.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[keturn]

When I wrote the first pass of this, I left out DeepSeek's option for response_format: json_object. Why would I ever want an LLM to pretend it's using structured JSON?

Then I found out about Ideogram 4.