feat: add an append only mode to add messages reducer

[jdrogers940]

• Description: Adding an optional mode parameter to the add_messages function with two modes:
  • allow_everything which is the default and does everything as normal today.
  • append_only which throws a ValueError if a user tries to either remove or adjust any existing messages.
• Issue: Solves the problem of a user who should have permissions to call a graph doing something to the previous history of the messages.
• Dependencies: None
• Twitter handle: N/A