Client-side display aliases for rooma/users

I'd like to see a standardized way to store local user remarks (client-side display aliases) in Matrix, allowing users to assign custom names to contacts or rooms without modifying server-side  display_name  data. Similar features exist in other platforms:

- XMPP
- Telegram
- Signal
- Wechat

Secret storage can be utilised: https://spec.matrix.org/v1.14/client-server-api/#secrets

Start with the linked Matrix client-server API secrets section and compare how the issue describes aliases for users and rooms. Define the required specification scope, storage and privacy behavior, and compatibility expectations before identifying the relevant specification sections to change.

Written by the indexing model from the issue text.