Support resuming actors with golden memory snapshot and DurableDirectory

[zoez7]

In the Actor Upgrades discussion during summit, we defined a rule that
"Substrate cannot upgrade into an existing memory snapshot".

When resuming an actor after ActorTemplateVersion upgrade, we will need to use

1. The memory portion from the GoldenSnapshot of the upgraded ActorTemplate Version and
2. The DurableDirectory from the previous run.

[zoez7]

Thinking about it more, I think this could be useful as a SnapshotScope type of its own

substrate/pkg/api/v1alpha1/actortemplate_types.go

Lines 253 to 265 in e1fa004

	// SnapshotScope defines what components to include in a snapshot.
	// +kubebuilder:validation:Enum=Full;Data
	type SnapshotScope string
	const (
	// Full captures process memory plus the entire filesystem delta on top of
	// the OCI image (including any attached DurableDir volumes).
	SnapshotScopeFull SnapshotScope = "Full"
	// Data captures only the contents of attached volumes that support
	// snapshots (currently DurableDir-typed volumes). Process memory and
	// the rest of rootfs are excluded.
	SnapshotScopeData SnapshotScope = "Data"
	)

not just as "magic" used during upgrades. ("As a substrate user, I want to use memory snapshot only for the purpose of avoiding cold boot")

But if we drop the idea of "mandatory golden".... having a GOLDEN_MEMORY_WITH_DUR_DIR enum value won't make much sense. cc: Julian Gutierrez Oschmann (@juli4n) Eitan Yarmush (@EItanya)