Add labels to the state

`Labels` is used to store some runtime specified informations,
these informations may be meaningless to other runtime, but it's
userful for the same runtime to operate this container.

Signed-off-by: Gao feng <omarapazanadi@gmail.com>

config.go

@@ -67,4 +67,6 @@ type State struct {
 	Pid int `json:"pid"`
 	// BundlePath is the path to the container's bundle directory.
 	BundlePath string `json:"bundlePath"`
+	// Lables is the runtime itself specified information.
+	Labels map[string]string `json:"labels"`
 }

runtime.md

@@ -22,6 +22,8 @@ This allows the hooks to perform cleanup and teardown logic after the runtime de
 * **`pid`**: (int) is the ID of the main process within the container, as seen by the host.
 * **`bundlePath`**: (string) is the absolute path to the container's bundle directory.
 This is provided so that consumers can find the container's configuration and root filesystem on the host.
+* **`Labels`**: (map) Labels is the runtime itself specified information.
+This allows runtime to store some informations which are usefull for itself but meaningless for other runtime.
 
 *Example*