feat: Add top-level parent logic to Upstream/Dowstream links [FC-0097] (#37076)

- Adds the `top_level_parent_usage_key` to the `EntityLinkBase`
- This field is used to save the top-level parent of a component or container when it is imported into a course. Example: A unit with components imported into a course. The unit is the top-level parent of the components.
- Updates the `DownstreamListView` to return the top-level parents instead of downstream child, if this parent exists.
- Each time containers with children were synchronized, a new downstream block was created for each child instead of updating the existing one. This occurred because the `upstream_key` was incorrectly validated as an `Opaquekey` against a list of key strings. This was fixed by converting the `upstream_key` to a string before the verification. (see 34cd5a4781 and 29647831dc)
- Which edX user roles will this change impact?  "Course Author", "Developer".

cms/lib/xblock/upstream_sync.py

@@ -25,7 +25,7 @@ from opaque_keys import InvalidKeyError
 from opaque_keys.edx.keys import CourseKey
 from opaque_keys.edx.locator import LibraryContainerLocator, LibraryUsageLocatorV2
 from xblock.exceptions import XBlockNotFoundError
-from xblock.fields import Scope, String, Integer
+from xblock.fields import Scope, String, Integer, Dict
 from xblock.core import XBlockMixin, XBlock
 
 if t.TYPE_CHECKING:
@@ -327,6 +327,17 @@ class UpstreamSyncMixin(XBlockMixin):
         default=None, scope=Scope.settings, hidden=True, enforce_type=True,
     )
 
+    top_level_downstream_parent_key = Dict(
+        help=(
+            "The block key ('block_type@block_id') of the downstream block that is the top-level parent of "
+            "this block. This is present if the creation of this block is a consequence of "
+            "importing a container that has one or more levels of children. "
+            "This represents the parent (container) in the top level "
+            "at the moment of the import."
+        ),
+        default=None, scope=Scope.settings, hidden=True, enforce_type=True,
+    )
+
     @classmethod
     def get_customizable_fields(cls) -> dict[str, str | None]:
         """