This PR introduces the following incompatible API changes to the Instance and Flavor Plugins:

New Label() Method Requirement for Instance Plugin

Addition of a new Label() method requirement on the Instance Plugin SPI. This adds a new requirement for Instance plugins to implement method for labeling / tagging a resource:

type Plugin interface {
    // existing methods
    Label(instance instance.ID, labels map[string]string) error
}

• This new method is necessary to support features for InfraKit to "import" (or assimilate) resources that have been created outside of InfraKit.
• It is also needed to ensure bootstrapping node is labeled properly to address some issues with rolling updates.

Use *types.Any instead of *json.RawMessage

• Replace *json.RawMessage in the methods of Flavor and Instance plugin methods with *types.Any.
• Makes consistent use of pointers in method parameters throughout and standardizes on a single abstraction of types.Any to represent user-defined, opaque structs.
• Removes lots of boilerplate code for JSON unmarshalling.
• This will make it easier to support other formats (e.g. YAML or GRPC) since we only need to change the encoding and decoding of data in a single location.

INSTANCE PLUGIN DEVELOPERS
Instead of the following boilerplate code

func (p *myPlugin) Validate(spec *json.RawMessage) error {
    mySpec := someStruct{}
    err := json.Unmarshal([]byte(*spec), &mySpec)
    if err != nil {
       return err
    }
    // ....  Do validate
}    

Use this instead:

func (p *myPlugin) Validate(spec *types.Any) error {
    mySpec := someStruct{}
    err := spec.Decode(&mySpec)
    if err != nil {
       return err
    }
    // ....  Do validate
}