feat: support x-machines placement constraints in compose files (#90)

internal/machine/api/proxy/backend.go

@@ -2,10 +2,10 @@ package proxy
 
 import (
 	"fmt"
+	"github.com/psviderski/uncloud/internal/machine/api/pb"
 	"google.golang.org/grpc/status"
 	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/proto"
-	"github.com/psviderski/uncloud/internal/machine/api/pb"
 )
 
 // One2ManyResponder converts upstream responses into messages from upstreams, so that multiple