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/test_signer.go

@@ -0,0 +1,27 @@
+package authentication
+
+// TestSigner represents an authentication key signer which we can use for
+// testing purposes only. This will largely be a stub to send through client
+// unit tests.
+type TestSigner struct{}
+
+// NewTestSigner constructs a new instance of test signer
+func NewTestSigner() (Signer, error) {
+	return &TestSigner{}, nil
+}
+
+func (s *TestSigner) DefaultAlgorithm() string {
+	return ""
+}
+
+func (s *TestSigner) KeyFingerprint() string {
+	return ""
+}
+
+func (s *TestSigner) Sign(dateHeader string) (string, error) {
+	return "", nil
+}
+
+func (s *TestSigner) SignRaw(toSign string) (string, string, error) {
+	return "", "", nil
+}