Add Support for Manta Object Storage as a Gateway (#5025)

Manta is an Object Storage by [Joyent](https://www.joyent.com/)

This PR adds initial support for Manta. It is intended as non-production 
ready so that feedback can be obtained.

vendor/github.com/joyent/triton-go/storage/signing.go

@@ -0,0 +1,81 @@
+package storage
+
+import (
+	"bytes"
+	"fmt"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/hashicorp/errwrap"
+)
+
+// SignURLInput represents parameters to a SignURL operation.
+type SignURLInput struct {
+	ValidityPeriod time.Duration
+	Method         string
+	ObjectPath     string
+}
+
+// SignURLOutput contains the outputs of a SignURL operation. To simply
+// access the signed URL, use the SignedURL method.
+type SignURLOutput struct {
+	host       string
+	objectPath string
+	Method     string
+	Algorithm  string
+	Signature  string
+	Expires    string
+	KeyID      string
+}
+
+// SignedURL returns a signed URL for the given scheme. Valid schemes are
+// `http` and `https`.
+func (output *SignURLOutput) SignedURL(scheme string) string {
+	query := &url.Values{}
+	query.Set("algorithm", output.Algorithm)
+	query.Set("expires", output.Expires)
+	query.Set("keyId", output.KeyID)
+	query.Set("signature", output.Signature)
+
+	sUrl := url.URL{}
+	sUrl.Scheme = scheme
+	sUrl.Host = output.host
+	sUrl.Path = output.objectPath
+	sUrl.RawQuery = query.Encode()
+
+	return sUrl.String()
+}
+
+// SignURL creates a time-expiring URL that can be shared with others.
+// This is useful to generate HTML links, for example.
+func (s *StorageClient) SignURL(input *SignURLInput) (*SignURLOutput, error) {
+	output := &SignURLOutput{
+		host:       s.Client.MantaURL.Host,
+		objectPath: fmt.Sprintf("/%s%s", s.Client.AccountName, input.ObjectPath),
+		Method:     input.Method,
+		Algorithm:  strings.ToUpper(s.Client.Authorizers[0].DefaultAlgorithm()),
+		Expires:    strconv.FormatInt(time.Now().Add(input.ValidityPeriod).Unix(), 10),
+		KeyID:      fmt.Sprintf("/%s/keys/%s", s.Client.AccountName, s.Client.Authorizers[0].KeyFingerprint()),
+	}
+
+	toSign := bytes.Buffer{}
+	toSign.WriteString(input.Method + "\n")
+	toSign.WriteString(s.Client.MantaURL.Host + "\n")
+	toSign.WriteString(fmt.Sprintf("/%s%s\n", s.Client.AccountName, input.ObjectPath))
+
+	query := &url.Values{}
+	query.Set("algorithm", output.Algorithm)
+	query.Set("expires", output.Expires)
+	query.Set("keyId", output.KeyID)
+	toSign.WriteString(query.Encode())
+
+	signature, _, err := s.Client.Authorizers[0].SignRaw(toSign.String())
+	if err != nil {
+		return nil, errwrap.Wrapf("Error signing string: {{err}}", err)
+	}
+
+	output.Signature = signature
+	return output, nil
+}