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/signature.go

@@ -0,0 +1,27 @@
+package authentication
+
+import (
+	"regexp"
+	"fmt"
+)
+
+type httpAuthSignature interface {
+	SignatureType() string
+	String() string
+}
+
+func keyFormatToKeyType(keyFormat string) (string, error) {
+	if keyFormat == "ssh-rsa" {
+		return "rsa", nil
+	}
+
+	if keyFormat == "ssh-ed25519" {
+		return "ed25519", nil
+	}
+
+	if regexp.MustCompile("^ecdsa-sha2-*").Match([]byte(keyFormat)) {
+		return "ecdsa", nil
+	}
+
+	return "", fmt.Errorf("Unknown key format: %s", keyFormat)
+}