object: Update verify routine

Verify routine now supports public key field to check
if integrity header contains correct signature of
object header.

Verify also have strict check that integrity header is
last header in the list of headers.

object/extensions.go

@@ -1,4 +1,5 @@
 package object
+// todo: all extensions must be transferred to the separate util library
 
 import "github.com/nspcc-dev/neofs-proto/storagegroup"