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### _Why are the changes needed?_
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After this PR, the service named service discovery will be a child service of frontend service and it's optional.
A servalable will contain a backend and a list of frontends, the former one stay AS-IS and the latter is changed from single to multiple, which means a serverable can have multiple interfaces to interact with its different clients
### _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
- [x] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#1144 from yaooqinn/1143.
Closes#1143
f2beb721 [Kent Yao] fix test
128cf282 [Kent Yao] nit
37aaff9c [Kent Yao] lift FrontendService trait
add335c4 [Kent Yao] nit
3a111cba [Kent Yao] [KYUUBI #1143] Refact Serverable/FrontendService/ServiceDiscovery hierarchy
bfa63b81 [Kent Yao] [KYUUBI #1143] Refact Serverable/FrontendService/ServiceDiscovery hierarchy
7e5a5cbb [Kent Yao] Merge branch 'master' into 1143
09263bc0 [Kent Yao] [KYUUBI #1143] Refact Serverable/FrontendService/ServiceDiscovery hierarchy
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>
0ee8eff5a0