Changing readers to async iterators

[alex-eri]

Is your feature request related to a problem?

Not problem, but looks legacy

while True:
    chunk = await field.read_chunk()
    if not chunk:
         break

or

    reader = await request.multipart()
    while True:
        field = await reader.next()
        if field is None: break

Describe the solution you'd like

async for chunk in field.chunks():
   f.write(chunk)

or

async for field in (await request.multipart()):
    ....

Describe alternatives you've considered

Or fix docs on objects already has anext interface.

Related component

Server

Additional context

No response

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