[REQ] [typescript-node] Promise-based Authentication interface

Created by: silasbw

Is your feature request related to a problem? Please describe.

The current TypeScript Node Authentication interface for applying authentication settings (.applyToRequest) is synchronous. It would be useful to support a Promise-based asynchronous API also. For example, to fetch a refreshed authentication token.

https://github.com/kubernetes-client/javascript/issues/294#issuecomment-510323971 is one example that would use this feature.

Describe the solution you'd like

Update Authentication.applyToRequest to return Promise<void> | void.

Describe alternatives you've considered

1. "spawnSync-style hacks", like serializing arguments and passing them to an script and parsing the result
2. Breaking change to Authentication.applyToRequest that forces all implementers to return a Promise<void>