Make plugin init function async

[rejifald]

Describe the topic
As an upload plugin developer, I would like to have ability to perform asynchronous operation while plugin initialization. For example, I have created simple plugin to upload files to Google Cloud Storage, and I need to make a request to the server, to check if a specified bucket exists, and create it if needed.

Your suggestions for this topic
Add support for async init function in provider upload plugins.

I am open to contribute.