[KYUUBI apache#5050] Add admin checking in getProxyUser

### _Why are the changes needed?_

Closes apache#5050.
Check the realUser is an admin or not before verifying proxy access.

### _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.readthedocs.io/en/master/contributing/code/testing.html#running-tests) locally before make a pull request

Closes apache#5051 from HaoYang670/5050_check_admin_in_get_proxy_user.

Closes apache#5050

cdabd58 [remzi] add admin check

Authored-by: remzi <[email protected]>
Signed-off-by: fwang12 <[email protected]>

kyuubi-server/src/main/scala/org/apache/kyuubi/server/KyuubiRestFrontendService.scala

@@ -235,7 +235,9 @@ class KyuubiRestFrontendService(override val serverable: Serverable)
       realUser
     } else {
       sessionConf.get(KyuubiAuthenticationFactory.HS2_PROXY_USER).map { proxyUser =>
-        KyuubiAuthenticationFactory.verifyProxyAccess(realUser, proxyUser, ipAddress, hadoopConf)
+        if (!getConf.get(KyuubiConf.SERVER_ADMINISTRATORS).contains(realUser)) {
+          KyuubiAuthenticationFactory.verifyProxyAccess(realUser, proxyUser, ipAddress, hadoopConf)
+        }
         proxyUser
       }.getOrElse(realUser)
     }