Async Authentication interface

[merlimat]

[lhotari]

This is a broader change and I'll first create a PIP. The Autentication interfaces are a mess in Pulsar and hopefully we can get them cleaned up for v5 client. There's a possibility to create a bridge that makes migration for third party implementations seamless when they can continue to be used.
The broker side interfaces are also a mess and that could be covered separately in another PIP.

[lhotari]

resolved by PIP-478 changes on the client side. The broker side will be separate.