clustermesh: Introduce CiliumClusterConfig

Add a new type CiliumClusterConfig which represents a cluster
configuration. This will be serialized and stored into kvstore during
the clustermesh-apiserver startup time. Later on, cilium-agent on each
node reads it when connecting to new clusters.

The current use case of this is getting ClusterID at connect time, but by
exposing the cluster configuration, we can also do some useful validation
such as

- Make sure the cluster id is not conflicting with existing clusters.
- Make sure the new cluster doesn't have any capability mismatch.

Signed-off-by: Yutaro Hayakawa <yutaro.hayakawa@isovalent.com>

pkg/clustermesh/types/types.go

@@ -10,3 +10,7 @@ const (
 	// ClusterIDMax is the maximum value of the cluster ID
 	ClusterIDMax = 255
 )
+
+type CiliumClusterConfig struct {
+	ID uint32 `json:"id,omitempty"`
+}

pkg/kvstore/kvstore.go

@@ -39,6 +39,9 @@ const (
 	// the heartbeat
 	HeartbeatPath = BaseKeyPrefix + "/.heartbeat"
 
+	// ClusterConfigPrefix is the kvstore prefix to cluster configuration
+	ClusterConfigPrefix = BaseKeyPrefix + "/cluster-config"
+
 	// HeartbeatWriteInterval is the interval in which the heartbeat key at
 	// HeartbeatPath is updated
 	HeartbeatWriteInterval = time.Minute