protobuf: remove gogoproto

Removes gogo/protobuf from buildx and updates to a version of
moby/buildkit where gogo is removed.

This also changes how the proto files are generated. This is because
newer versions of protobuf are more strict about name conflicts. If two
files have the same name (even if they are relative paths) and are used
in different protoc commands, they'll conflict in the registry.

Since protobuf file generation doesn't work very well with
`paths=source_relative`, this removes the `go:generate` expression and
just relies on the dockerfile to perform the generation.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

vendor/google.golang.org/grpc/internal/transport/proxy.go

@@ -107,8 +107,14 @@ func doHTTPConnectHandshake(ctx context.Context, conn net.Conn, backendAddr stri
 		}
 		return nil, fmt.Errorf("failed to do connect handshake, response: %q", dump)
 	}
-
-	return &bufConn{Conn: conn, r: r}, nil
+	// The buffer could contain extra bytes from the target server, so we can't
+	// discard it. However, in many cases where the server waits for the client
+	// to send the first message (e.g. when TLS is being used), the buffer will
+	// be empty, so we can avoid the overhead of reading through this buffer.
+	if r.Buffered() != 0 {
+		return &bufConn{Conn: conn, r: r}, nil
+	}
+	return conn, nil
 }
 
 // proxyDial dials, connecting to a proxy first if necessary. Checks if a proxy