docs: Improve annotations in "Context: Existing Fields" examples

[sobolevn]

Link: https://faststream.airt.ai/latest/getting-started/context/existed/#access-to-context-fields

This code:

@broker_object.subscriber("test-topic")
async def handle(
    msg: str,
    logger=Context(),
    message=Context(),
    broker=Context(),
    context=Context(),
):

Feels a bit off, because there are no annotations for all things fetched from Context. I think that we should annotate these fields as real types that will be fetched from Context. But, I am not sure what they return, so, I can't annotate them.