Proposal to use a room as Hotline (clients can only see their own messages)

**Is your feature request related to a problem? Please describe.**
I would like to use Matrix to manage a Hotline. But with the current permission model, this is not possible to guarantee privacy for users requesting help.

**Describe the solution you'd like**
A single room where all support staff and clients can write messages.
But a client can only see his own messages and the answers from support staff (in a Thread, meaning a message is handled like a ticket where staff and client can exchange on the same topic).

**Describe alternatives you've considered**
A single room where everybody can see the messages from others, but for obvious privacy reasons, it is not possible.
Or every users can start a direct chat with a member of the staff, but the staff will be quickly overwhelmed.

**Additional context**
Setting up Matrix for a large organization, I wanted to provide support for the users through Matrix.

No files, tests, or entry points are named. Begin by reviewing the Matrix permission model and threading requirements described in the proposal; done would require a specified privacy-preserving room behavior and an agreed implementation scope.

Written by the indexing model from the issue text.