[#525] core/container: Define removal witness

Define `RemovalWitness` structure which groups the information required to
prove and verify the removal of a container. This type is going to be used
in container Delete-related methods.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/core/container/delete.go

@@ -0,0 +1,35 @@
+package container
+
+import (
+	cid "github.com/nspcc-dev/neofs-api-go/pkg/container/id"
+)
+
+// RemovalWitness groups the information required
+// to prove and verify the removal of a container.
+type RemovalWitness struct {
+	cid *cid.ID
+
+	sig []byte
+}
+
+// ContainerID returns identifier of the container
+// to be removed.
+func (x RemovalWitness) ContainerID() *cid.ID {
+	return x.cid
+}
+
+// SetContainerID sets identifier of the container
+// to be removed.
+func (x *RemovalWitness) SetContainerID(id *cid.ID) {
+	x.cid = id
+}
+
+// Signature returns signature of the container identifier.
+func (x RemovalWitness) Signature() []byte {
+	return x.sig
+}
+
+// SetSignature sets signature of the container identifier.
+func (x *RemovalWitness) SetSignature(sig []byte) {
+	x.sig = sig
+}