Add support for required plugins.

Signed-off-by: Lantao Liu <lantaol@google.com>

services/server/config/config.go

@@ -39,6 +39,9 @@ type Config struct {
 	// DisabledPlugins are IDs of plugins to disable. Disabled plugins won't be
 	// initialized and started.
 	DisabledPlugins []string `toml:"disabled_plugins"`
+	// RequiredPlugins are IDs of required plugins. Containerd exits if any
+	// required plugin doesn't exist or fails to be initialized or started.
+	RequiredPlugins []string `toml:"required_plugins"`
 	// Plugins provides plugin specific configuration for the initialization of a plugin
 	Plugins map[string]toml.Primitive `toml:"plugins"`
 	// OOMScore adjust the containerd's oom score