admin: Add Background heal status info API (#7774)

This API returns the information related to the self healing routine.

For the moment, it returns:
- The total number of objects that are scanned
- The last time when an item was scanned

cmd/peer-rest-common.go

@@ -26,6 +26,7 @@ const (
 	peerRESTMethodDrivePerfInfo            = "driveperfinfo"
 	peerRESTMethodDeleteBucket             = "deletebucket"
 	peerRESTMethodSignalService            = "signalservice"
+	peerRESTMethodBackgroundHealStatus     = "backgroundhealstatus"
 	peerRESTMethodGetLocks                 = "getlocks"
 	peerRESTMethodBucketPolicyRemove       = "removebucketpolicy"
 	peerRESTMethodLoadUser                 = "loaduser"