Support for serialised data in raccoon client

[punit-kulal]

Summary
Currently, the raccoon, client supports in built serialization for either PROTO(ProtoMessage Object), or JSON. This means for already serialised data like in case for users of Stencil, they have to explicitly parse the message into a DynamicMessage for it to be of serializer Proto. We need to add support for a Serializer.RAW which forwards the message as it is without explicitly parsing the message they have serialised.

Proposed solution
Introduce Serialiser.RAW which gives the input as it is after checking for type safety

// SerializerFunc defines a conversion for raccoon message to byte sequence.
type SerializerFunc func(interface{}) ([]byte, error)

var (
	// json raccoon message serializer
	JSON = json.Marshal

	// proto raccoon message serializer
	PROTO = func(m interface{}) ([]byte, error) {
		msg, ok := m.(proto.Message)
		if !ok {
			return nil, errors.New("unable to marshal non proto")
		}
		return proto.Marshal(msg)
	}

      // New serialiser type proposed.
       RAW = func(m interface{}) ([]byte, error) {
	        typedM, ok := m.([]byte)
	        if !ok {
		        return nil, errors.New("failed to convert to byte array")
	        }
	       return typedM, nil
       } 
	
)

Additional context
Currently for stencil users,

        stencilUrl := fmt.Sprintf(stencilSchemaTemplate, namespace, schemaName)
	client, err := stencil.NewClient([]string{stencilUrl}, stencil.Options{})
	if err != nil {
		return nil, err
	}
	serialisedData, err := client.Serialize(packageName, data) // is of type []byte
	protoMessage, err := client.Parse(packageName, serialisedData) // additional step for converting the data to a protoMessage.
	if err != nil {
		return nil, err
	}
	return protoMessage, err

instead of directly returning the serialisedData.