private void handleExtensionsValidatorCallback(OAuthBearerExtensionsValidatorCallback extensionsValidatorCallback) {
        checkInitialized();

        extensionsValidatorCallback.inputExtensions().map().forEach((extensionName, v) -> extensionsValidatorCallback.valid(extensionName));
    }
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