Introduce executeSql method in TableEnvironment

interface TableEnvironment {
     /**
      * Execute the given single statement and
      * the statement can be DDL/DML/DQL/SHOW/DESCRIBE/EXPLAIN/USE.
      *
      * If the statement is translated to a Flink job (DML/DQL),
      * the TableResult will be returned until the job is submitted, and
      * contains a JobClient instance to associate the job.
      * Else, the TableResult will be returned until the statement
      * execution is finished, does not contain a JobClient instance.
      *
      * @return result for DQL/SHOW/DESCRIBE/EXPLAIN, the affected row count
      * for `DML` (-1 means unknown),or a string message ("OK") for other 
      * statements.
      */
    TableResult executeSql(String statement);
}

/**
 * A TableResult is the representation of the statement execution result.
 */
interface TableResult {
   /**
    * return JobClient if a Flink job is submitted
    * (for DML/DQL statement), else return empty (e.g. DDL).
    */
   Optional<JobClient> getJobClient();
 
   /** 
    * Get the schema of result. 
    */
    TableSchema getTableSchema();
     
    /**
     * return the ResultKind which can avoid custom parsing of
     * an "OK" row in programming
     */
    ResultKind getResultKind();
 
     /**
      * Get the result contents as an iterable rows.
      */
    Iterator<Row> collect();
 
    /**
     * Print the result contents.
     */
    void print();
}

/**
 * ResultKind defines the types of the result.
 */
public enum ResultKind {
    // for DDL, DCL and statements with a simple "OK"
    SUCCESS,
 
    // rows with important content are available (DML, DQL)
    SUCCESS_WITH_CONTENT
}