[REQ] Support for a bearer token supplier for Java RestTemplate clients

[GregDThomas]

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

Under certain circumstances it is useful to be able to provide a supplier for bearer tokens with using a RestTemplate 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.

[GregDThomas]

(working on a fix for this)