Query option to dump TExecRequest

To investigate planner bugs like IMPALA-13274, it'd be helpful to check the TExecRequest of the query. However, enabling TRACE level logs of Frontend doesn't dump it. There is a flag, dump_exec_request_path, which will let coordinator dump the TExecRequest to files under the given folder. That's a startup flag that requires a restart and will dump TExecRequests for all queries.

A query option that lets coordinator output the TExecRequest to the client without executing the query will be helpful. Note that the query might cause crash so we don't want it to be executed. Explaining the query will get a different TExecRequest.