[SPARK-49007][CORE] Improve MasterPage to support custom title

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

This PR aims to improve `MasterPage` to support custom title.

### Why are the changes needed?

When there exists multiple Spark clusters, custom title can be more helpful than the spark master address because it can contain semantics like the role of the clusters. In addition, the URL field in the same page already provides the spark master information even when we use a custom title.

**BEFORE**
```
sbin/start-master.sh
```
![Screenshot 2024-07-25 at 14 01 11](https://github.com/user-attachments/assets/7055d700-4bd6-4785-a535-2f8ce6dba47d)

**AFTER**
```
SPARK_MASTER_OPTS='-Dspark.master.ui.title="Projext X Staging Cluster"' sbin/start-master.sh
```
![Screenshot 2024-07-25 at 14 05 38](https://github.com/user-attachments/assets/f7e45fd6-fa2b-4547-ae39-1403b1e910d9)

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

### How was this patch tested?

Pass the CIs with newly added test case.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#47491 from dongjoon-hyun/SPARK-49007.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/main/scala/org/apache/spark/deploy/master/ui/MasterPage.scala

@@ -26,11 +26,13 @@ import org.apache.spark.deploy.DeployMessages.{KillDriverResponse, MasterStateRe
 import org.apache.spark.deploy.JsonProtocol
 import org.apache.spark.deploy.StandaloneResourceUtils._
 import org.apache.spark.deploy.master._
+import org.apache.spark.internal.config.UI.MASTER_UI_TITLE
 import org.apache.spark.ui.{UIUtils, WebUIPage}
 import org.apache.spark.util.Utils
 
 private[ui] class MasterPage(parent: MasterWebUI) extends WebUIPage("") {
   private val master = parent.masterEndpointRef
+  private val title = parent.master.conf.get(MASTER_UI_TITLE)
   private val jsonFieldPattern = "/json/([a-zA-Z]+).*".r
 
   def getMasterState: MasterStateResponse = {
@@ -266,7 +268,7 @@ private[ui] class MasterPage(parent: MasterWebUI) extends WebUIPage("") {
           }
         </div>;
 
-    UIUtils.basicSparkPage(request, content, "Spark Master at " + state.uri)
+    UIUtils.basicSparkPage(request, content, title.getOrElse("Spark Master at " + state.uri))
   }
 
   private def workerRow(showResourceColumn: Boolean): WorkerInfo => Seq[Node] = worker => {

core/src/main/scala/org/apache/spark/internal/config/UI.scala

@@ -244,6 +244,13 @@ private[spark] object UI {
     .checkValues(Set("ALLOW", "LOCAL", "DENY"))
     .createWithDefault("LOCAL")
 
+  val MASTER_UI_TITLE = ConfigBuilder("spark.master.ui.title")
+    .version("4.0.0")
+    .doc("Specifies the title of the Master UI page. If unset, `Spark Master at <MasterURL>` " +
+      "is used by default.")
+    .stringConf
+    .createOptional
+
   val UI_SQL_GROUP_SUB_EXECUTION_ENABLED = ConfigBuilder("spark.ui.groupSQLSubExecutionEnabled")
     .doc("Whether to group sub executions together in SQL UI when they belong to the same " +
       "root execution")

core/src/test/scala/org/apache/spark/deploy/master/MasterWorkerUISuite.scala

@@ -140,4 +140,22 @@ class MasterWorkerUISuite extends MasterSuiteBase {
       System.getProperties().remove("spark.ui.proxyBase")
     }
   }
+
+  test("SPARK-49007: Support custom master web ui title") {
+    implicit val formats = org.json4s.DefaultFormats
+    val title = "Spark Custom Title"
+    val conf = new SparkConf().set(MASTER_UI_TITLE, title)
+    val localCluster = LocalSparkCluster(2, 2, 512, conf)
+    localCluster.start()
+    val masterUrl = s"http://${Utils.localHostNameForURI()}:${localCluster.masterWebUIPort}"
+    try {
+      eventually(timeout(50.seconds), interval(100.milliseconds)) {
+        val html = Utils
+          .tryWithResource(Source.fromURL(s"$masterUrl/"))(_.getLines().mkString("\n"))
+        html should include (title)
+      }
+    } finally {
+      localCluster.stop()
+    }
+  }
 }