[feature request] Support Alikernel isolation options for Pouch

[allencloud]

Hi, All,

We need to make alibaba/runc vendored in Pouch could take advantage of Alikernel's open sourced container isolation options.

These options should be passed by Pouchd, and be carried in github.com/opencontainers/runtime-spec/specs-go. The specific carrier is the Annotation field in type Spec:

// Spec is the base configuration for the container.
type Spec struct {
        ......
	Hooks *Hooks `json:"hooks,omitempty" platform:"linux,solaris"`
	// Annotations contains arbitrary metadata for the container.
	Annotations map[string]string `json:"annotations,omitempty"`
        ......
}

All these four options in recorded in pr AliyunContainerService/pouch#743. We made a rule that these options are put into Annotation by a specified Key and Value:

• Key: a prefix of _, and has a _ between every word;
• Value: convert that into a string type.

When we finished transferring data via annotation to runC, then in the implementation of runC could extract data in annotation and deal with these options to configure Alikernel options related to pouch container.