public Pair<SaslClientAuthenticationProvider, Token<? extends TokenIdentifier>>
    selectProvider(String clusterId, User clientUser) {
    return selector.selectProvider(clusterId, clientUser);
  }
Chooses the best authentication provider and corresponding token given the HBase cluster
identifier and the user.
hbase-client/src/main/java/org/apache/hadoop/hbase/security/provider/SaslClientAuthenticationProviders.java