Add message queue agent (#6463)

* scaffold message queue agent

* adjust proto, add mq_agent

* add agent client implementation

* remove unused function

* agent publish server implementation

* adding agent

weed/mq/client/agent_client/agent_subscribe.go

@@ -0,0 +1,17 @@
+package agent_client
+
+import (
+	"github.com/seaweedfs/seaweedfs/weed/pb/schema_pb"
+)
+
+func (a *SubscribeSession) SubscribeMessageRecord(
+	onEachMessageFn func(key []byte, record *schema_pb.RecordValue),
+	onCompletionFn func()) error {
+	for {
+		resp, err := a.stream.Recv()
+		if err != nil {
+			return err
+		}
+		onEachMessageFn(resp.Key, resp.Value)
+	}
+}