Add staging_path to NodeExpand/GetStats calls

Allow COs to explicitly pass the staging path to node plugins when
invoking NodeGetVolumeStats and NodeExpandVolume. Previously the
CO could choose to pass either the staging or publish path, but
there was no way for the node plugin to easily know which path it
was.

This change allows COs to pass both paths (if it wishes) or just
the staging path, and in either case, the node plugin can easily
figure out what it's being given. This makes its significantly
easier for node plugins to implement the NodeGetVolumeStats and
NodeExpandVolume RPCs, in particular it relieves the need to
store some state on the node.

For backwards compatibility, the field is not required, but it
should be trivially easy for any CO to populate the field, so we
encourage clients to set it.

Release Note:
```
Added optional staging_target_path field to NodeExpandVolume()
and NodeGetVolumeStats(). CO implementers should populate this
new field if possible when calling these RPCs.
```

Author: bswartz

csi.proto

@@ -1213,6 +1213,13 @@ message NodeGetVolumeStatsRequest {
   // the process serving this request.
   // This is a REQUIRED field.
   string volume_path = 2;
+
+  // The path where the volume is staged, if the plugin has the
+  // STAGE_UNSTAGE_VOLUME capability, otherwise empty.
+  // If not empty, it MUST be an absolute path in the root
+  // filesystem of the process serving this request.
+  // This field is OPTIONAL.
+  string staging_path = 3;
 }
 
 message NodeGetVolumeStatsResponse {
@@ -1327,6 +1334,13 @@ message NodeExpandVolumeRequest {
   // plugin MAY expand the volume to its maximum capacity.
   // This field is OPTIONAL.
   CapacityRange capacity_range = 3;
+
+  // The path where the volume is staged, if the plugin has the
+  // STAGE_UNSTAGE_VOLUME capability, otherwise empty.
+  // If not empty, it MUST be an absolute path in the root
+  // filesystem of the process serving this request.
+  // This field is OPTIONAL.
+  string staging_path = 4;
 }
 
 message NodeExpandVolumeResponse {