Add support for async requests

[jimmyjames]

Changes

Fixes #315

This change adds the ability to execute a request asynchronously, through the addition of a new default method on the Request interface:

default CompletableFuture<T> executeAsync()

The default interface implementation throws an UnsupportedOperationException. The implementation in BaseRequest returns a CompletableFuture. An example usage:

api.users().list(null).executeAsync().whenComplete((usersPage, ex) -> {
    if (ex != null) {
        System.out.println("Exception occurred: " + ex.getMessage());
    } else {
        System.out.println("Users list size: " + usersPage.getItems().size());
    }
}).get();

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors