Checklist

• I have checked the issues list
  for similar or identical feature requests.
• I have checked the pull requests list
  for existing proposed implementations of this feature.
• I have checked the commit log
  to find out if the if the same feature was already implemented in the
  master branch.
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  in this issue (If there are none, check this box anyway).

Brief Summary

Support async celery tasks & call async function in celery tasks

Design

async def foo(x, y):
    return x + y

@app.task(
    name='tasks.add',
    queue='custom',
    rate_limit='10/s',
)
async def add(x, y):
    await foo(x, y)