Support for custom RDBMS is broken

HIVE-24120 introduced code to support custom RDBMS.

DatabaseProduct.getDbType(String productName) will return DbType.UNDEFINED for anything other than the hardcoded/internally supported database types.

When initializing DatabaseProduct with an external/custom RDBMS, it follows this logic:

        boolean isExternal = MetastoreConf.getBoolVar(conf, ConfVars.USE_CUSTOM_RDBMS);

        if (isExternal) {
          // The DatabaseProduct will be created by instantiating an external class via
          // reflection. The external class can override any method in the current class
          String className = MetastoreConf.getVar(conf, ConfVars.CUSTOM_RDBMS_CLASSNAME);

          if (className != null) {
            try {
              theDatabaseProduct = (DatabaseProduct)
                  ReflectionUtils.newInstance(Class.forName(className), conf);

              LOG.info(String.format("Using custom RDBMS %s", className));
              dbt = DbType.CUSTOM;

These 2 database types (DbType.UNDEFINED, DbType.CUSTOM) are then compared to each other to make sure they are the same.

Preconditions.checkState(theDatabaseProduct.dbType == getDbType(productName));

https://github.com/gatorblue/hive/blob/3a65c6cf9cb552e7c34bfb449a419abfde0a58b6/standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/metastore/DatabaseProduct.java#L80