Notifications for threads proof of concept

[germain-gg]

Implement proof of concept for matrix-org/matrix-spec-proposals#3773

[germain-gg]

Screengrab of the proof of concept for MSC3773.

Screen.Recording.2022-08-18.at.15.56.27-720.mov

To run using the Netlify deployment of this PR matrix-org/matrix-react-sdk#9130 and the threads dev server deployed by @clokep.
It also includes the read receipt proof of concept, #22980

[germain-gg]

Closing as we managed to reach a conclusion with this proof of concept