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>
