fix: Add Chat-Group-Name-Timestamp header and use it to update group names (#6412)

Add "Chat-Group-Name-Timestamp" message header and use the last-write-wins logic when updating group
names (similar to group member timestamps). Note that if the "Chat-Group-Name-Changed" header is
absent though, we don't add a system message (`MsgGrpNameChangedBy`) because we don't want to blame
anyone.

src/headerdef.rs

@@ -57,6 +57,7 @@ pub enum HeaderDef {
     ChatGroupId,
     ChatGroupName,
     ChatGroupNameChanged,
+    ChatGroupNameTimestamp,
     ChatVerified,
     ChatGroupAvatar,
     ChatUserAvatar,