[Cluster] Enable HeartbeatResponse message type

[Arkatufus]

Fixes #6061

Changes

• Enable HeartbeatResponse and Heartbeat protobuf wire format for cluster hearbeats.
• Add akka.cluster.use-legacy-heartbeat-message HOCON settings, defaults to false. When set to true, the serializer will use the old legacy Heartbeat messages.
• Add HeartbeatResponse and Heartbeat proto serializer

[Aaronontheweb]

Need to change our defaults, but otherwise LGTM

[Arkatufus]

Done, changed the setting to opt-out, not opt-in

[Aaronontheweb]

I think we can skip the serializer changes and just modify

 private T AssertAndReturn<T>(T message)
        {
            var serializer = Sys.Serialization.FindSerializerFor(message);
            var serialized = serializer.ToBinary(message);
            serializer.Should().BeOfType<ClusterMessageSerializer>();
            return serializer.FromBinary<T>(serialized);
        }

to cast the returned serializer into SerializerWithStringManifest and then manually call .Manifest on that. If the serialization binding is wrong then the cast will fail.

[Aaronontheweb]

Need to harden tests

[Aaronontheweb]

This test only succeeds because -1,, -1 are the hard-coded values when legacy==on. Need to harden this to make sure that the correct, non--1 values are supported when legacy==off.

[Aaronontheweb]

The AssertEqual regardless of config is what's not going to work here. Change

 var message = new ClusterHeartbeatSender.HeartbeatRsp(uniqueAddress, -1, -1);

to

 var message = new ClusterHeartbeatSender.HeartbeatRsp(uniqueAddress, 10, 3);

When legacy off, 10,3

When legacy on, -1, -1

[Arkatufus]

Done

[Aaronontheweb]

LGTM - nice work @Arkatufus

[Aaronontheweb]

TODO: need to add an issue for this letting users know about this setting as part of the Akka.NET 1.5 upgrade advisories