Asynchronous jwt sign

[henrybyhee]

Sorry for the late response.

I believe some users would like to inject their own Token provider and in some instance, signing jwt is an asynchronous process. For instance, signing jwt via a cloud provider is asynchronous if user stores private key on cloud service and it's not possible to be retrieved as plain text

Originally posted by @Henry-bee in #70 (comment)

[AndreyKishkevich]

We have the same issue, to be more security with Fireblocks private key we want to generate a token from AWS KMS cloud service and this service is an asynchronous.