Allow Refresh/Update of Management Api Token

[sean-91]

As it stands there appears to be no easy way to refresh the apiToken of a ManagementApi. Since the token used here has an expiration, a new ManagementApi requires instantiating with a new valid token when this occurs.

The problem arises with the use of the OkHttpClient instantiated with a ManagementApi. This leads to a memory leak, as each successive ManagementApi creates a new OkHttpClient that is never cleaned up.

A similar issue was addressed here: square/okhttp#2846

A setter for the apiToken would alleviate this issue, allowing it to be updated without the need to re-instantiate the ManagementApi

[lbalmaceda]

Yes, that makes sense. Thanks for raising this up. The new api token would be used on the next call to any of the entities' methods, since it's passed on that call e.g. https://github.com/auth0/auth0-java/blob/master/src/main/java/com/auth0/client/mgmt/ManagementAPI.java#L160