@Override
    public void close() {
        if (verificationKeyResolver != null) {
            try {
                verificationKeyResolver.close();
            } catch (Exception e) {
                log.error(e.getMessage(), e);
            }
        }
    }
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