Add support of SubModel instead of SubSettings

[jag-k]

Code:

from pydantic import BaseModel, Field

from pydantic_settings import BaseSettings, SettingsConfigDict


class SubModel(BaseModel):
    val: int = 0
    flag: bool = False


class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_nested_delimiter='__')

    nested_model: SubModel = Field(default_factory=SubModel)

Now:

# Configuration

Here you can find all available configuration options using ENV variables.

## Settings

Settings.

| Name           | Type       | Default                  | Description | Example                  |
|----------------|------------|--------------------------|-------------|--------------------------|
| `NESTED_MODEL` | `SubModel` | `{"val":0,"flag":false}` |             | `{"val":0,"flag":false}` |

Want to:

# Configuration

Here you can find all available configuration options using ENV variables.

## Settings

Settings.

### SubModel

SubModel.

**Environment Prefix**: `NESTED_MODEL__`

| Name                 | Type      | Default | Description | Example |
|----------------------|-----------|---------|-------------|---------|
| `NESTED_MODEL__VAL`  | `integer` | `0`     |             | `0`     |
| `NESTED_MODEL__FLAG` | `boolean` | `false` |             | `false` |