Add subPath support for PVC mounts

- Added sub_path field to PvcMount protobuf message
- Updated mount_pvc() function to accept optional sub_path parameter
- Added test case for mounting PVC with subPath
- Fixes #11832

Author: Mridul verma

kubernetes_platform/proto/kubernetes_executor_config.proto

@@ -93,6 +93,9 @@ message PvcMount {
     // Container path to which the PVC should be mounted.
     string mount_path = 4;
 
+    // Optional subPath inside the PVC to mount (instead of mounting the volume root).
+    string sub_path = 6;
+
     // Name of the PVC.
     ml_pipelines.TaskInputsSpec.InputParameterSpec pvc_name_parameter = 5;
 }