Add transfer proxy client

Signed-off-by: Derek McGowan <derek@mcg.dev>

pkg/transfer/proxy/transfer.go

@@ -0,0 +1,63 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package proxy
+
+import (
+	"context"
+
+	transferapi "github.com/containerd/containerd/api/services/transfer/v1"
+	"github.com/containerd/containerd/pkg/transfer"
+	"github.com/containerd/typeurl"
+	"google.golang.org/protobuf/types/known/anypb"
+)
+
+type proxyTransferer struct {
+	client transferapi.TransferClient
+}
+
+// NewTransferer returns a new transferr which communicates over a GRPC
+// connection using the containerd transfer API
+func NewTransferer(client transferapi.TransferClient) transfer.Transferer {
+	return &proxyTransferer{
+		client: client,
+	}
+}
+
+func (p *proxyTransferer) Transfer(ctx context.Context, src interface{}, dst interface{}, opts ...transfer.Opt) error {
+	asrc, err := typeurl.MarshalAny(src)
+	if err != nil {
+		return err
+	}
+	adst, err := typeurl.MarshalAny(dst)
+	if err != nil {
+		return err
+	}
+	// Resolve opts to
+	req := &transferapi.TransferRequest{
+		Source: &anypb.Any{
+			TypeUrl: asrc.GetTypeUrl(),
+			Value:   asrc.GetValue(),
+		},
+		Destination: &anypb.Any{
+			TypeUrl: adst.GetTypeUrl(),
+			Value:   adst.GetValue(),
+		},
+		// TODO: Options
+	}
+	_, err = p.client.Transfer(ctx, req)
+	return err
+}