Support free-form JSON body params for models

[alexey-kudryavtsev]

I'm using Page Assist mainly with OpenRouter API. Some models there require model-specific JSON body params to do useful things. For instance, x.ai grok models can search and cite web and tweets if you pass them tools parameter like

  "tools": [
    {
      "type": "web_search",
      "enable_image_understanding": true
    },
    {
      "type": "x_search",
      "allowed_x_handles": ["elonmusk"]
    }
  ]

as described on https://docs.x.ai/docs/guides/tools/search-tools.
Openrouter passes them through correctly. There are other cases like adding "reasoning" param that is supported by Openrouter itself (documented at https://openrouter.ai/docs/guides/best-practices/reasoning-tokens). And of course this is not limited to Openrouter, other providers often make use of vendor-specific params to steer model behavior.
It would be great if page assist had support for merging free-form JSON (or adding custom params with JSON values) per model on top of existing standard options (temperature, max tokens, top p, etc):