Support async function

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.
• I have included all related issues and possible duplicate issues 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)