Support MSC4028 (Push all encrypted events except for muted rooms) #15847

Description

The purpose of MSC4028 is to be able to support on mobile clients the "mentions-and-keywords-only" room notification setting in encrypted rooms. This MSC adds a new "server-default" override push rule (.m.rule.encrypted_event) in order to force the server to push all encrypted events to the registered client except for muted room.

I request here to add this new "server-default" push rule with its unstable id org.matrix.msc4028.encrypted_event. Contrary to the stable push rule, this unstable one should not be enabled -> its enabled attribute must be false. Indeed we should not turn it on until the clients implementations are ready.

I listed here the potential issues of this MSC server side. We talked about them in an internal discussion, the conclusion was:

So calculating what to push and the unread counts (its the same process) is a big performance bottleneck currently when sending in large rooms. A bunch of optimisation has taken place to make that better, but its still quite expensive
I'm not so concerned about /sync, as we've mostly already done the work at that point

I would let the backend team use this issue or create new ones to handle any other dev related to this MSC.