Add Eden AI as a provider

Hi, thanks a lot for aichat, it is really nice to use.

I would like to propose adding Eden AI as a built in provider.

A bit of context on why:

• We have users who would like to reach Eden AI through aichat.
• Eden AI is a European company, and that is the main point for us. It has an EU data residency endpoint, zero data retention, a DPA, and SOC 2 and ISO 27001. For teams in the EU that is often what makes the difference.
• Eden AI is OpenAI compatible. One API key reaches models from many providers (OpenAI, Anthropic, Google, Mistral, Cohere and more), with ids like "openai/gpt-4o-mini", the same provider/model shape openrouter already uses.

Because it is OpenAI compatible, it fits the existing openai-compatible client. It just needs one entry in OPENAI_COMPATIBLE_PROVIDERS (base url https://api.edenai.run/v3), a models block in models.yaml, and the usual config.example.yaml and Argcfile.sh entries. No new client code.

We are on the Eden AI team and would maintain it. I already have a working implementation that builds, passes fmt and the tests, and I tested it live against the real API (a normal call and a streaming call, across OpenAI and Anthropic models). Happy to open the PR right away.

Would you be open to this?