[REQ] Support for a oauth access token supplier for Java RestClient clients

[twonky4]

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

Under certain circumstances it is useful to be able to provide a supplier for oauth access tokens with using a RestClient based Java client.

Describe the solution you'd like

The ApiClient currently supports setAccessToken(String) method - this is extended so that a setAccessToken(Supplier<String>) can be invoked.

Additional context

This was already implemented for RestTemplate OAuth or RestClient HttpBearerAuth.

[twonky4]

(working on a fix for this)