Add 'runtime' to the state

There are some points we can get benefit from the 'runtime' field,

1, clearly get the runtime of container, it's useful for statistic.
2, different kinds of runtime will not try to operate incompatible container,
   such as, runc should not execute(runc exec) program on runv container.

Signed-off-by: Gao feng <[email protected]>

Author: gao-feng

config.go

@@ -67,4 +67,6 @@ type State struct {
 	Pid int `json:"pid"`
 	// Root is the path to the container's bundle directory.
 	Root string `json:"root"`
+	// Runtime is the runtime of OCI container
+	Runtime string `json:"runtime"`
 }

runtime.md

@@ -13,6 +13,7 @@ By providing a default location that container state is stored external applicat
 * **id** (string) ID is the container's ID.
 * **pid** (int) Pid is the ID of the main process within the container.
 * **root** (string) Root is the path to the container's bundle directory.
+* **runtime** (string) Runtime is the runtime of OCI container.
 
 The ID is provided in the state because hooks will be executed with the state as the payload.
 This allows the hook to perform clean and teardown logic after the runtime destroys its own state.
@@ -25,7 +26,8 @@ The root directory to the bundle is provided in the state so that consumers can
 {
     "id": "oc-container",
     "pid": 4422,
-    "root": "/containers/redis"
+    "root": "/containers/redis",
+    "runtime": "runc"
 }
 ```