Support submit job with different executor at runtime

[DashShen]

Currently, flink-sql-gateway can only submit job with a specified executor which defined in flink-conf.yaml。But if someone wants to submit a job to yarn-session, and others want to do it with standalone cluster, then flink-sql-gateway can't satisfy this right now. So could flink-sql-gateway support this feature future。

And I also has a roughly idea that how flink-sql-gateway supports this feature. we can introduce a new component which I called ExecutionManager .ExecutionManager is like SessionManager, but it's main function is to creating a new Execution, each Execution represents an deploy mode(yarn-seesion, yarn-per-job, standalone, local, etc). The Execution will create a new SessionManager and will use user specified execution.target to instance a DefaultContext. By this way, every new session created by SessionManager can submit job to different cluster.

Furthermore, flink-sql-gateway do not need to create SessionManager at start time but instead of ExecutionManager, and the SessionManager will be instance by ExecutionManager.

And, we'll must implement a new netty handler that used to create/delete/get Execution.