Allows users to forward providerOptions to Vercel's ai package. This is
particularly useful for openrouter when needing to select specific providers.

This allows the user to follow the documentation on providerOptions 1:1 with a
given providers sdk.

Example

Following this documentation for adding provider options to the body of a request, we can use
the following opencode.json to select a specific provider

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "openrouter": {
      "npm": "@openrouter/ai-sdk-provider",
      "name": "OpenRouter",
      "options": {},
      "models": {
        "deepseek/deepseek-r1:free": {
          "tools": false,
          "id": "deepseek/deepseek-r1:free",
          "name": "DeepSeek: R1 (free)",
          "providerOptions": {
            "openrouter": {
              "provider": {
                "allow_fallbacks": false,
                "only": [
                  "targon"
                ]
              }
            }
          }
        }
      }
    }
  }
}

Notes

I added the tools field to the model info as an optional bool, since
openrouter providers may or may not support tools (in my case, Targon does not
yet support tool calling), so this needs to be dynamic per model. Maybe it makes
sense to name this something closer to disableTools?

Closes #186