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/rsa_signature.go

@@ -0,0 +1,25 @@
+package authentication
+
+import (
+	"encoding/base64"
+)
+
+type rsaSignature struct {
+	hashAlgorithm string
+	signature     []byte
+}
+
+func (s *rsaSignature) SignatureType() string {
+	return s.hashAlgorithm
+}
+
+func (s *rsaSignature) String() string {
+	return base64.StdEncoding.EncodeToString(s.signature)
+}
+
+func newRSASignature(signatureBlob []byte) (*rsaSignature, error) {
+	return &rsaSignature{
+		hashAlgorithm: "rsa-sha1",
+		signature:     signatureBlob,
+	}, nil
+}