add Guest Star collaboration contract

contracts/v1/collaboration-session-response.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": ["object", "null"],
+  "additionalProperties": false,
+  "required": ["id", "guests"],
+  "properties": {
+    "id": { "type": "string" },
+    "guests": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "additionalProperties": false,
+        "required": ["user_id"],
+        "properties": {
+          "user_id": { "type": "string" },
+          "user_display_name": { "type": ["string", "null"] },
+          "slot_id": { "type": ["string", "null"] },
+          "is_live": { "type": ["boolean", "null"] }
+        }
+      }
+    }
+  }
+}

fixtures/v1/collaboration-session.response.json

@@ -0,0 +1,4 @@
+{
+  "id": "guest-star-session-1",
+  "guests": []
+}