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### _Why are the changes needed?_
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Now kyuubi supports SQL and SCALA language.
Now the way to switch OPERATION_LANAUAGE:
```
// for SCALA
spark.sql("set kyuubi.operation.language=SCALA")
// for SQL
set kyuubi.operation.language=SCALA
```
It is more user friendly that user can specify the OPERATION_LANGUAGE for each ExecuteStatement.
### _How was this patch tested?_
- [x] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [ ] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#1710 from turboFei/conf_overlay.
Closes#1710
385264dc [Fei Wang] fix ut
58fbdbdf [Fei Wang] fix npe
2718fb48 [Fei Wang] Support to get OPERATION_LANGUAGE from execute statement confOverlay
Authored-by: Fei Wang <fwang12@ebay.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
e0eeab07eb