text: Title: Always include `additionalProperties: True` for dictionary schemas
  Body: ### Initial Checks
  
  - [x] I have searched Google & GitHub for similar requests and couldn't find anything
  - [x] I have read and followed [the docs](https://docs.pydantic.dev) and still think this feature is missing
  
  ### Description
  
  Currently for a model like this
  ```py
  from typing import Any, Dict
  from pydantic import BaseModel
  
  class Model(BaseModel):
      conditions: Dict[str, Any]
  
  print(Model.model_json_schema())
  ```
  Pydantic will generate this JSON Schema
  ```json
  {
    "properties": {
      "conditions": {
        "title": "Conditions",
        "type": "object"
      }
    },
    "required": [
      "conditions"
    ],
    "title": "Model",
    "type": "object"
  }
  ```
  
  I think it'd be a nice improvement to include `additionalProperties: True`, even though it is the spec default as it just makes the schemas easier to read / parse / validate imo, e.g.:
  ```json
  {
    "title": "Conditions",
    "type": "object",
    "additionalProperties": true,
  }
  ```
  
  I'm not aware of any downsides that being explicit here would bring? Apart from the potential for breaking changes?
  
  ### Affected Components
  
  - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
  - [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)
  - [ ] [Data serialization](https://docs.pydantic.dev/concepts/serialization/) - `.model_dump()` and `.model_dump_json()`
  - [x] [JSON Schema](https://docs.pydantic.dev/concepts/json_schema/)
  - [ ] [Dataclasses](https://docs.pydantic.dev/concepts/dataclasses/)
  - [ ] [Model Config](https://docs.pydantic.dev/concepts/config/)
  - [ ] [Field Types](https://docs.pydantic.dev/api/types/) - adding or changing a particular data type
  - [ ] [Function validation decorator](https://docs.pydantic.dev/concepts/validation_decorator/)
  - [ ] [Generic Models](https://docs.pydantic.dev/concepts/models/#generic-models)
  - [ ] [Other Model behaviour](https://docs.pydantic.dev/concepts/models/) - `model_construct()`, pickling, private attributes, ORM mode
  - [ ] [Plugins](https://docs.pydantic.dev/) and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.