private static void handleCallback(SaslExtensionsCallback extensionsCallback, Subject subject) {
        if (subject != null && !subject.getPublicCredentials(SaslExtensions.class).isEmpty()) {
            SaslExtensions extensions = subject.getPublicCredentials(SaslExtensions.class).iterator().next();
            extensionsCallback.extensions(extensions);
        }
    }
Attaches the first {@link SaslExtensions} found in the public credentials of the Subject
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/OAuthBearerSaslClientCallbackHandler.java