@Override
    public void handle(Callback[] callbacks) throws IOException, UnsupportedCallbackException {
        checkInitialized();

        for (Callback callback : callbacks) {
            if (callback instanceof OAuthBearerValidatorCallback) {
                handleValidatorCallback((OAuthBearerValidatorCallback) callback);
            } else if (callback instanceof OAuthBearerExtensionsValidatorCallback) {
                handleExtensionsValidatorCallback((OAuthBearerExtensionsValidatorCallback) callback);
            } else {
                throw new UnsupportedCallbackException(callback);
            }
        }
    }
Because a {@link CloseableVerificationKeyResolver} instance can spawn threads and issue
HTTP(S) calls ({@link RefreshingHttpsJwksVerificationKeyResolver}), we only want to create
a new instance for each particular set of configuration. Because each set of configuration
may have multiple instances, we want to reuse the single instance.
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/OAuthBearerValidatorCallbackHandler.java