[SPARK-44849] Expose SparkConnectExecutionManager.listActiveExecutions

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

Make list of active executions accessible via SparkConnectService.listActiveExecutions

### Why are the changes needed?

Some internal components outside `connect` would like to have access to that. Add a method to expose it.

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

No.

### How was this patch tested?

CI

Closes #42535 from juliuszsompolski/SPARK-44849.

Authored-by: Juliusz Sompolski <julek@databricks.com>
Signed-off-by: Herman van Hovell <herman@databricks.com>

connector/connect/server/src/main/scala/org/apache/spark/sql/connect/service/SparkConnectService.scala

@@ -348,6 +348,12 @@ object SparkConnectService extends Logging {
     userSessionMapping.get((userId, sessionId), default)
   }
 
+  /**
+   * If there are no executions, return Left with System.currentTimeMillis of last active
+   * execution. Otherwise return Right with list of ExecuteInfo of all executions.
+   */
+  def listActiveExecutions: Either[Long, Seq[ExecuteInfo]] = executionManager.listActiveExecutions
+
   /**
    * Used for testing
    */