### _Why are the changes needed?_
After some thought and some offline discussions, I propose to change the configuration key from `kyuubi.engine.pool.balance.policy` to `kyuubi.engine.pool.selectPolicy`.
And in the future if we introduce properties for specific policy, e.g. supposing we have a engine selection policy named `LOAD`, the properties would be
```
kyuubi.engine.pool.selectPolicy.load.abc=xxxx
kyuubi.engine.pool.selectPolicy.load.xyz=xxxx
```
### _How was this patch tested?_
- [ ] 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.apache.org/docs/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#3966 from pan3793/rename.
Closes#2887
7c1eda70 [Cheng Pan] address comments
bf3a3863 [Cheng Pan] [KYUUBI #2887][FOLLOWUP] Rename configuration to kyuubi.engine.pool.selectPolicy
Authored-by: Cheng Pan <chengpan@apache.org>
Signed-off-by: Cheng Pan <chengpan@apache.org>