bp: #38203
1. Previously, if the root path of the HDFS URI started with two
slashes, the outfile would be successfully exported without errors, but
the exported path would not be the expected path.
Currently, we will delete repeated '/' which specified by users in FE.
2. move the test case for outfile HDFS from p2 to p0.
In the original logic, the `Export` statement generates `Selectstmt` for execution. But there is no way to make the `SelectStmt` use the new optimizer.
Now, we change the `Export` statement to generate the `outfile SQL`, and then use the new optimizer to parse the SQL so that outfile can use the new optimizer.
If there is only one fragment of a query plan, FE will call `exec_plan_fragment` rpc to BE.
And on BE side, the `exec_plan_fragment()` will be executed directly in bthread, but it may call
some JNI method like `AttachCurrentThread()`, which will return error in bthread.
So I modify the `exec_plan_fragment` to make sure it will be executed in pthread pool.
This pr does three things:
1. add `delete_existing_files` property for outfile/export. If `delete_existing_files = true`, export/outfile will delete all files under file_path first.
2. add p2 test for export
3. modify docs
