Python: add async 'for' loop support to LogScanner

[fresh-borzoni]

Search before asking

• I searched in the issues and found nothing similar.

Description

LogScanner should implement aiter/anext so users can write smth like this:

  async for record in scanner:
      process(record)

Instead of the current manual poll loop:

  while True:
      records = scanner.poll(1000)
      for record in records:
          process(record)

the same for record_batch_log_scanner and log_scanner, they differ in iterable types, but overall that's the idea

Willingness to contribute

• I'm willing to submit a PR!