Accelerate Maven artifact repository uploads

This MR implements Accelerate uploads for gitlab-org/gitlab-ee!6607 implementing https://gitlab.com/gitlab-org/gitlab-ee/issues/5811

Maven uploads files using the message body (like LFS). here we are extracting a configurable BodyUploader that we can use for both kind of uploads

internal/filestore/body_uploader.go

+package filestore
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"gitlab.com/gitlab-org/gitlab-workhorse/internal/api"
+	"gitlab.com/gitlab-org/gitlab-workhorse/internal/helper"
+)
+
+type PreAuthorizer interface {
+	PreAuthorizeHandler(next api.HandleFunc, suffix string) http.Handler
+}
+
+// UploadVerifier allows to check an upload before sending it to rails
+type UploadVerifier interface {
+	// Verify can abort the upload returning an error
+	Verify(handler *FileHandler) error
+}
+
+// UploadPreparer allows to customize BodyUploader configuration
+type UploadPreparer interface {
+	// Prepare converts api.Response into a *SaveFileOpts, it can optionally return an UploadVerifier that will be
+	// invoked after the real upload, before the finalization with rails
+	Prepare(a *api.Response) (*SaveFileOpts, UploadVerifier, error)
+}
+
+type defaultPreparer struct{}
+
+func (s *defaultPreparer) Prepare(a *api.Response) (*SaveFileOpts, UploadVerifier, error) {
+	return GetOpts(a), nil, nil
+}
+
+// BodyUploader is an http.Handler that perform a pre authorization call to rails before hijacking the request body and
+// uploading it.
+// Providing an UploadPreparer allows to customize the upload process
+func BodyUploader(rails PreAuthorizer, h http.Handler, p UploadPreparer) http.Handler {
+	if p == nil {
+		p = &defaultPreparer{}
+	}
+
+	return rails.PreAuthorizeHandler(func(w http.ResponseWriter, r *http.Request, a *api.Response) {
+		opts, verifier, err := p.Prepare(a)
+		if err != nil {
+			helper.Fail500(w, r, fmt.Errorf("BodyUploader: preparation failed: %v", err))
+			return
+		}
+
+		fh, err := SaveFileFromReader(r.Context(), r.Body, r.ContentLength, opts)
+		if err != nil {
+			helper.Fail500(w, r, fmt.Errorf("BodyUploader: upload failed: %v", err))
+			return
+		}
+
+		if verifier != nil {
+			if err := verifier.Verify(fh); err != nil {
+				helper.Fail500(w, r, fmt.Errorf("BodyUploader: verification failed: %v", err))
+				return
+			}
+		}
+
+		data := url.Values{}
+		for k, v := range fh.GitLabFinalizeFields("file") {
+			data.Set(k, v)
+		}
+
+		// Hijack body
+		body := data.Encode()
+		r.Body = ioutil.NopCloser(strings.NewReader(body))
+		r.ContentLength = int64(len(body))
+		r.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+		// And proxy the request
+		h.ServeHTTP(w, r)
+	}, "/authorize")
+}