[SPARK-29087][CORE][STREAMING] Use DelegatingServletContextHandler to avoid CCE

### What changes were proposed in this pull request?

[SPARK-27122](#24088) fixes `ClassCastException` at `yarn` module by introducing `DelegatingServletContextHandler`. Initially, this was discovered with JDK9+, but the class path issues affected JDK8 environment, too. After [SPARK-28709](#25439), I also hit the similar issue at `streaming` module.

This PR aims to fix `streaming` module by adding `getContextPath` to `DelegatingServletContextHandler` and using it.

### Why are the changes needed?

Currently, when we test `streaming` module independently, it fails like the following.
```
$ build/mvn test -pl streaming
...
UISeleniumSuite:
- attaching and detaching a Streaming tab *** FAILED ***
  java.lang.ClassCastException: org.sparkproject.jetty.servlet.ServletContextHandler cannot be cast to org.eclipse.jetty.servlet.ServletContextHandler
...
Tests: succeeded 337, failed 1, canceled 0, ignored 1, pending 0
*** 1 TEST FAILED ***
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
```

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

Pass the Jenkins with the modified tests. And do the following manually.
Since you can observe this when you run `streaming` module test only (instead of running all), you need to install the changed `core` module and use it.

```
$ java -version
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_222-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.222-b10, mixed mode)
$ build/mvn install -DskipTests
$ build/mvn test -pl streaming
```

Closes #25791 from dongjoon-hyun/SPARK-29087.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 729b318)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: dongjoon-hyun

core/src/main/scala/org/apache/spark/ui/WebUI.scala

@@ -229,4 +229,8 @@ private[spark] class DelegatingServletContextHandler(handler: ServletContextHand
   def filterCount(): Int = {
     handler.getServletHandler.getFilters.length
   }
+
+  def getContextPath(): String = {
+    handler.getContextPath
+  }
 }

streaming/src/test/scala/org/apache/spark/streaming/UISeleniumSuite.scala

@@ -96,7 +96,7 @@ class UISeleniumSuite
 
       val sparkUI = ssc.sparkContext.ui.get
 
-      sparkUI.getHandlers.count(_.getContextPath.contains("/streaming")) should be (5)
+      sparkUI.getDelegatingHandlers.count(_.getContextPath.contains("/streaming")) should be (5)
 
       eventually(timeout(10 seconds), interval(50 milliseconds)) {
         go to (sparkUI.webUrl.stripSuffix("/"))
@@ -197,7 +197,7 @@ class UISeleniumSuite
 
       ssc.stop(false)
 
-      sparkUI.getHandlers.count(_.getContextPath.contains("/streaming")) should be (0)
+      sparkUI.getDelegatingHandlers.count(_.getContextPath.contains("/streaming")) should be (0)
 
       eventually(timeout(10 seconds), interval(50 milliseconds)) {
         go to (sparkUI.webUrl.stripSuffix("/"))