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/signer.go

@@ -0,0 +1,10 @@
+package authentication
+
+const authorizationHeaderFormat = `Signature keyId="%s",algorithm="%s",headers="%s",signature="%s"`
+
+type Signer interface {
+	DefaultAlgorithm() string
+	KeyFingerprint() string
+	Sign(dateHeader string) (string, error)
+	SignRaw(toSign string) (string, string, error)
+}