Feature request: Role-based channel access

I'm shocked that nothing like this has been implemented or even proposed.

It seems to me that at least basic role-based channel access is a must. You can have a look at how Discord does it for a comprehensive implementation, but here's the simplest implementation I can think of:

Owners should be able to create roles and assign them to users, and to add "tags" to channels. And then there should be a way to modulate the access level (Reader, Writer, Admin, Owner) per-channel depending on the role of the user and the tags assigned to the channel.

A new access level called "Observer" should be added, which is below Reader and grants read-only access to channels.

The issue names no files, tests, or entry points. Start by mapping the existing channel permissions and access levels, then clarify the role, tag, and Observer behavior with maintainers; done requires an agreed design and corresponding coverage for the access rules.

Written by the indexing model from the issue text.