plugin XEP-0277: add mentions to extra data:

rel 463

libervia/backend/plugins/plugin_xep_0277.py

@@ -139,6 +139,8 @@ class MbExtra(BaseModel):
     encrypted: bool = False
     encrypted_for: dict | None = None
     signed: bool = False
+    # FIXME: Must be moved to XEP-0372 plugin.
+    mentions: list[JIDType] = Field(default_factory=list)
 
 
 class MbData(BaseModel):