leases: support resource management

Add three methods for lease service so that the client can use it to
manage the resource by lease, not just gc.root label. With the following
methods, it is easy for client to maintain their own cache system.

```
 - AddResource(context.Context, Lease, Resource) error
 - RemoveResource(context.Context, Lease, Resource) error
 - ListResources(context.Context, Lease) ([]Resource, error)
```

And the resource is to be

```golang
type Resource {
  ID   string
  Type string
}
```

For the snapshots, the Type field will be formatted by
snapshots/%{type}, like snapshots/overlayfs.

fix: #3295

Signed-off-by: Wei Fu <fuweid89@gmail.com>

leases/lease.go

@@ -32,6 +32,9 @@ type Manager interface {
 	Create(context.Context, ...Opt) (Lease, error)
 	Delete(context.Context, Lease, ...DeleteOpt) error
 	List(context.Context, ...string) ([]Lease, error)
+	AddResource(context.Context, Lease, Resource) error
+	DeleteResource(context.Context, Lease, Resource) error
+	ListResources(context.Context, Lease) ([]Resource, error)
 }
 
 // Lease retains resources to prevent cleanup before
@@ -42,6 +45,13 @@ type Lease struct {
 	Labels    map[string]string
 }
 
+// Resource represents low level resource of image, like content, ingest and
+// snapshotter.
+type Resource struct {
+	ID   string
+	Type string
+}
+
 // DeleteOptions provide options on image delete
 type DeleteOptions struct {
 	Synchronous bool

leases/proxy/manager.go

@@ -91,3 +91,43 @@ func (pm *proxyManager) List(ctx context.Context, filters ...string) ([]leases.L
 
 	return l, nil
 }
+
+func (pm *proxyManager) AddResource(ctx context.Context, lease leases.Lease, r leases.Resource) error {
+	_, err := pm.client.AddResource(ctx, &leasesapi.AddResourceRequest{
+		ID: lease.ID,
+		Resource: leasesapi.Resource{
+			ID:   r.ID,
+			Type: r.Type,
+		},
+	})
+	return errdefs.FromGRPC(err)
+}
+
+func (pm *proxyManager) DeleteResource(ctx context.Context, lease leases.Lease, r leases.Resource) error {
+	_, err := pm.client.DeleteResource(ctx, &leasesapi.DeleteResourceRequest{
+		ID: lease.ID,
+		Resource: leasesapi.Resource{
+			ID:   r.ID,
+			Type: r.Type,
+		},
+	})
+	return errdefs.FromGRPC(err)
+}
+
+func (pm *proxyManager) ListResources(ctx context.Context, lease leases.Lease) ([]leases.Resource, error) {
+	resp, err := pm.client.ListResources(ctx, &leasesapi.ListResourcesRequest{
+		ID: lease.ID,
+	})
+	if err != nil {
+		return nil, errdefs.FromGRPC(err)
+	}
+
+	rs := make([]leases.Resource, 0, len(resp.Resources))
+	for _, i := range resp.Resources {
+		rs = append(rs, leases.Resource{
+			ID:   i.ID,
+			Type: i.Type,
+		})
+	}
+	return rs, nil
+}