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/storage/snaplink.go

@@ -0,0 +1,46 @@
+package storage
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/hashicorp/errwrap"
+	"github.com/joyent/triton-go/client"
+)
+
+type SnapLinksClient struct {
+	client *client.Client
+}
+
+// PutSnapLinkInput represents parameters to a PutSnapLink operation.
+type PutSnapLinkInput struct {
+	LinkPath   string
+	SourcePath string
+}
+
+// PutSnapLink creates a SnapLink to an object.
+func (s *SnapLinksClient) Put(ctx context.Context, input *PutSnapLinkInput) error {
+	linkPath := fmt.Sprintf("/%s%s", s.client.AccountName, input.LinkPath)
+	sourcePath := fmt.Sprintf("/%s%s", s.client.AccountName, input.SourcePath)
+	headers := &http.Header{}
+	headers.Set("Content-Type", "application/json; type=link")
+	headers.Set("location", sourcePath)
+	headers.Set("Accept", "~1.0")
+	headers.Set("Accept-Version", "application/json, */*")
+
+	reqInput := client.RequestInput{
+		Method:  http.MethodPut,
+		Path:    linkPath,
+		Headers: headers,
+	}
+	respBody, _, err := s.client.ExecuteRequestStorage(ctx, reqInput)
+	if respBody != nil {
+		defer respBody.Close()
+	}
+	if err != nil {
+		return errwrap.Wrapf("Error executing PutSnapLink request: {{err}}", err)
+	}
+
+	return nil
+}