Implement a VolumeEcStatus() RPC for volume servers. (#8006)

Just like `VolumeStatus()`, this call allows inspecting details for
a given EC volume - including number of files and their total size.

weed/storage/erasure_coding/ec_volume.go

@@ -333,6 +333,13 @@ func (ev *EcVolume) IsTimeToDestroy() bool {
 	return ev.ExpireAtSec > 0 && time.Now().Unix() > (int64(ev.ExpireAtSec)+destroyDelaySeconds)
 }
 
+func (ev *EcVolume) WalkIndex(processNeedleFn func(key types.NeedleId, offset types.Offset, size types.Size) error) error {
+	if ev.ecxFile == nil {
+		return fmt.Errorf("no ECX file associated with EC volume %v", ev.VolumeId)
+	}
+	return idx.WalkIndexFile(ev.ecxFile, 0, processNeedleFn)
+}
+
 func (ev *EcVolume) CheckIndex() (int64, []error) {
 	if ev.ecxFile == nil {
 		return 0, []error{fmt.Errorf("no ECX file associated with EC volume %v", ev.VolumeId)}