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/authentication/ecdsa_signature.go

@@ -0,0 +1,66 @@
+package authentication
+
+import (
+	"encoding/asn1"
+	"encoding/base64"
+	"fmt"
+	"math/big"
+
+	"github.com/hashicorp/errwrap"
+	"golang.org/x/crypto/ssh"
+)
+
+type ecdsaSignature struct {
+	hashAlgorithm string
+	R             *big.Int
+	S             *big.Int
+}
+
+func (s *ecdsaSignature) SignatureType() string {
+	return fmt.Sprintf("ecdsa-%s", s.hashAlgorithm)
+}
+
+func (s *ecdsaSignature) String() string {
+	toEncode := struct {
+		R *big.Int
+		S *big.Int
+	}{
+		R: s.R,
+		S: s.S,
+	}
+
+	signatureBytes, err := asn1.Marshal(toEncode)
+	if err != nil {
+		panic(fmt.Sprintf("Error marshaling signature: %s", err))
+	}
+
+	return base64.StdEncoding.EncodeToString(signatureBytes)
+}
+
+func newECDSASignature(signatureBlob []byte) (*ecdsaSignature, error) {
+	var ecSig struct {
+		R *big.Int
+		S *big.Int
+	}
+
+	if err := ssh.Unmarshal(signatureBlob, &ecSig); err != nil {
+		return nil, errwrap.Wrapf("Error unmarshaling signature: {{err}}", err)
+	}
+
+	rValue := ecSig.R.Bytes()
+	var hashAlgorithm string
+	switch len(rValue) {
+	case 31, 32:
+		hashAlgorithm = "sha256"
+	case 65, 66:
+		hashAlgorithm = "sha512"
+	default:
+		return nil, fmt.Errorf("Unsupported key length: %d", len(rValue))
+	}
+
+	return &ecdsaSignature{
+		hashAlgorithm: hashAlgorithm,
+		R:             ecSig.R,
+		S:             ecSig.S,
+	}, nil
+}