Add API for communicating with Docker volume plugins

Docker provides extensibility through a plugin system, of which
several types are available. This provides an initial library API
for communicating with one type of plugins, volume plugins.
Volume plugins allow for an external service to create and manage
a volume on Podman's behalf.

This does not integrate the plugin system into Libpod or Podman
yet; that will come in subsequent pull requests.

Signed-off-by: Matthew Heon <mheon@redhat.com>

vendor/github.com/docker/go-plugins-helpers/sdk/encoder.go

@@ -0,0 +1,37 @@
+package sdk
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+// DefaultContentTypeV1_1 is the default content type accepted and sent by the plugins.
+const DefaultContentTypeV1_1 = "application/vnd.docker.plugins.v1.1+json"
+
+// DecodeRequest decodes an http request into a given structure.
+func DecodeRequest(w http.ResponseWriter, r *http.Request, req interface{}) (err error) {
+	if err = json.NewDecoder(r.Body).Decode(req); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+	}
+	return
+}
+
+// EncodeResponse encodes the given structure into an http response.
+func EncodeResponse(w http.ResponseWriter, res interface{}, err bool) {
+	w.Header().Set("Content-Type", DefaultContentTypeV1_1)
+	if err {
+		w.WriteHeader(http.StatusInternalServerError)
+	}
+	json.NewEncoder(w).Encode(res)
+}
+
+// StreamResponse streams a response object to the client
+func StreamResponse(w http.ResponseWriter, data io.ReadCloser) {
+	w.Header().Set("Content-Type", DefaultContentTypeV1_1)
+	if _, err := copyBuf(w, data); err != nil {
+		fmt.Printf("ERROR in stream: %v\n", err)
+	}
+	data.Close()
+}