[PROPOSAL] Async ParDo API for Apache Beam

[Bharath Kumara Subramanian]

Hi all,

I have put a proposal
<https://docs.google.com/document/d/1t--UYXgaij0ULEoXUnhG3r8OZPBljN9r_WWlwQJBDrI/edit?usp=sharing>
to
add async ParDo API.

For context, please refer to the archive conversation:
https://www.mail-archive.com/dev@beam.apache.org/msg12101.html

Thanks,
Bharath