Add: structured output support via StructuredOutput type

[fswair]

Add: StructuredOutput decorator for structured output in Pydantic models
Add: Type alias structured for decorator usage convenience
Add: New entities to __all__ in output.py and _output.py
Add: Imports for new entities in __init__.py
Add: Tests for StructuredOutput with TestModel and decorator pattern

A factory type that enables structured output behavior in Pydantic models.

Example usage:

    from pydantic_ai import Agent
    from pydantic import BaseModel
    from pydantic_ai.output import structured
    from pydantic_ai.output import StructuredOutput
    
    # structured is a type alias for StructuredOutput
    # StructuredOutput(Model) is equivalent to @structured <ModelDef>
    
    @structured
    class FooBar(BaseModel):
        foo: str
        bar: int
    
    class BarBaz(BaseModel):
        bar: str
        baz: int
    
    agent  =  Agent()
    result =  agent.run_sync(
        "Generate a sample object",
        output_type=[FooBar, StructuredOutput(BarBaz)]
    )
    
    assert isinstance(result.output, dict)

[fswair]

Inspiration from: https://pydanticlogfire.slack.com/archives/C083V7PMHHA/p1747066464421339?thread_ts=1747058090.100679&cid=C083V7PMHHA