No OpenRouter integration

[doncat99]

It is suggested to add OpenRouter support. Then we can switch models more conveniently.

[aksg87]

Hey @doncat99, OpenRouter would work great as a provider plugin!

Since we just landed the provider registry (#97), the community can now create provider packages. See the discussion at #99 and check out examples/custom_provider_plugin/ for implementation patterns.

Let's continue the provider discussion in #99 - I'll close this issue to consolidate the conversation there.

[aksg87]

Closing to consolidate provider discussions in #99