P2P: allow to serialize UnknownCapability

Otherwise it can't be properly passed through the P2P network which makes
it impossible to use UnknownCapabilities as an extensions in the existing
networks.

Follow nspcc-dev/neo-go#3778.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

src/Neo/Network/P2P/Capabilities/UnknownCapability.cs

@@ -48,7 +48,7 @@ protected override void DeserializeWithoutType(ref MemoryReader reader)
 
         protected override void SerializeWithoutType(BinaryWriter writer)
         {
-            throw new InvalidOperationException("Unknown capability can't be serialized");
+            writer.WriteVarBytes(Data.Span);
         }
     }
 }

tests/Neo.UnitTests/Network/P2P/Capabilities/UT_UnknownCapability.cs

@@ -30,7 +30,7 @@ public void DeserializeUnknown()
             var capab = (NodeCapability)NodeCapability.DeserializeFrom(ref br);
 
             Assert.IsTrue(capab is UnknownCapability);
-            Assert.ThrowsException<InvalidOperationException>(() => capab.ToArray());
+            CollectionAssert.AreEqual(buffer, capab.ToArray());
         }
     }
 }