chore(evaluation-context): Support virtual segments (#6143)

Author: khvn26

sdk/evaluation-context.json

@@ -170,6 +170,12 @@
                     },
                     "title": "Overrides",
                     "type": "array"
+                },
+                "virtual": {
+                    "description": "Indicates whether the segment is virtual (i.e., not stored in Flagsmith but provided at evaluation time).",
+                    "title": "Virtual",
+                    "type": "boolean",
+                    "default": false
                 }
             },
             "required": [