pss: expose salt len in the verifyingkey (#448)

Some vendor serialization (NDA :() of signature and public keys will
need the size of the salt that was used for signature. Sadly this is
only exposed in the signing key (which may be out of reach (HSM)).

src/pss/verifying_key.rs

@@ -46,6 +46,11 @@ where
             phantom: Default::default(),
         }
     }
+
+    /// Return specified salt length for this key
+    pub fn salt_len(&self) -> usize {
+        self.salt_len
+    }
 }
 
 //