feat: add extra_bodyfield and use in model settings.

resolved: #1583

The `extra_body` field is a parameter used by the `openai` provider.

Since `litellm` also uses this field to additionally transmit `request body`, I added a function so that `aider` can also utilize the `extra_body` field.

The `openrouter` provider also supports various functions through the additional field of `request body`, so we added the function.

The following is how to use it in model settings.
```yaml
# .aider.model.settings.yml
- name: "openrouter/<YOUR_MODEL>"
  edit_format: "whole"
  use_repo_map: true
  extra_body:
    provider:
      order:
      - Azure
      allow_fallbacks: false
```

aider/coders/base_coder.py

@@ -1396,6 +1396,7 @@ class Coder:
                 self.stream,
                 temp,
                 extra_headers=model.extra_headers,
+                extra_body=model.extra_body,
                 max_tokens=model.max_tokens,
             )
             self.chat_completion_call_hashes.append(hash_object.hexdigest())