Implement MSC3952: Intentional mentions

[manuroe]

MSC3952 defines push rules which act on a list of MXIDs in the event body, instead of searching the entire event body for display name / local part.

There might be some more MSCs to implement to get it right. The related EW issue lists 3 others MSCs as dependencies.

As a SDK user, I need to:

• be notified when my app user is mentioned
• Allow clients to send intentional mentions #2722

[Velin92]

Kamino cloned this issue to matrix-org/matrix-rust-sdk

[Velin92]

This can be closed since technically speaking is the server supports MSC 4028, the user gets notified when they are mentioned