### _Why are the changes needed?_
Adapt to multiple frontends protocol.
This PR also marks REST API as experimental and exposes it to users.
### _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
- [x] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#1349 from pan3793/frontend-conf.
Closes#1349
d113c4a6 [Cheng Pan] Simplify code by SAM
129c2c9a [Cheng Pan] Fix compile
fa8f7f67 [Cheng Pan] Add new conf kyuubi.frontend.protocols
Authored-by: Cheng Pan <chengpan@apache.org>
Signed-off-by: Cheng Pan <chengpan@apache.org>
32c3f5cb2e