Round robin load balancer (#1873)

Motivation:

To build a grpc channel we need to support different load-balancers. One
of these is the round-robin load balancer which is added by this patch.

Modifications:

Add a `RoundRobinLoadBalancer` which maintains a set of subchannels
and picks between ready subchannels for RPCs using the round-robin
algorithm.

Result:

Can maintain a set of connections to a service using and pick between
them using round-robin

Sources/GRPCHTTP2Core/Client/Connection/LoadBalancers/LoadBalancerEvent.swift

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/// Events emitted by load-balancers.
+enum LoadBalancerEvent: Sendable, Hashable {
+  /// The connectivity state of the subchannel changed.
+  case connectivityStateChanged(ConnectivityState)
+  /// The subchannel requests that the load balancer re-resolves names.
+  case requiresNameResolution
+}