[Feature] Support Flink SQL Agent

[HamaWhiteGG]

Similar to Spark SQL Agent, visit https://python.langchain.com/docs/modules/agents/toolkits/spark_sql for more details.

[HamaWhiteGG]

I have implemented a local version FlinkSqlToolkitTest.

@Test
void testRunQuery() {
    var flinkSql = FlinkSql.builder()
            .tableEnv(tableEnv)
            .build()
            .init();

    var llm = ChatOpenAI.builder()
            .temperature(0)
            .build().init();

    var toolkit = new FlinkSqlToolkit(flinkSql, llm);
    var agentExecutor = createFlinkSqlAgent(llm, toolkit);

    // SELECT SQRT(AVG(Age)) FROM titanic
    var actual = agentExecutor.run("whats the square root of the average age?");
    // sometimes it's 'The square root of the average age is approximately 5.07.'
    assertEquals("5.07", actual);

    // TODO: It should be DESC here, not ASC.
    // SELECT Name FROM titanic WHERE Survived = 1 ORDER BY Age ASC LIMIT 1
    actual = agentExecutor.run("What's the name of the oldest survived passenger?");
    assertEquals("Sandstrom, Miss. Marguerite Rut", actual);
}