[#33] container: Define read-only storage interface

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

pkg/core/container/storage.go

@@ -0,0 +1,24 @@
+package container
+
+import (
+	"errors"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/container"
+)
+
+// Source is an interface that wraps
+// basic container receiving method.
+type Source interface {
+	// Get reads the container from the storage by identifier.
+	// It returns the pointer to requested container and any error encountered.
+	//
+	// Get must return exactly one non-nil value.
+	// Get must return ErrNotFound if the container is not in storage.
+	//
+	// Implementations must not retain the container pointer and modify
+	// the container through it.
+	Get(*container.ID) (*container.Container, error)
+}
+
+// ErrNotFound is the error returned when container was not found in storage.
+var ErrNotFound = errors.New("container not found")