[Prep] Threads: Design a participation model API in Matrix

[manuroe]

Description

We need a new CS API that manages in an efficient way user notications as we expect them to be in Threads 2.0.

This preparation work is mandatory for #2765, #2766 and #2767.

Acceptance criteria

• A MSC defines the new API
• Synapse implements it
• Matrix-rust-sdk implements it

Dependencies

• None

Out of scope

Subtasks

• Threads 2.0: Design and implement a thread participation/subscription model synapse#18431
• Threads: Implement the new participation model API matrix-org/matrix-rust-sdk#5038

[reivilibre]

There is now a draft MSC: MSC4306 for this.

[giomfo]

the initial implementation of 'MSC4306 except push rules' just landed in Synapse, in the merge window for the RC — element-hq/synapse#18674

[giomfo]

Note: MSC4308: Thread Subscriptions extension to Sliding Sync has been created to get a live-update of thread subscriptions

The implementation of this MSC at the Rust SDK level is done: matrix-org/matrix-rust-sdk#5604

@reivilibre can you please provide in this ticket an updated status from the server side