Add default push rule for sticker messages

Hi, I hope this belongs here. Please let me know if I need to create an MSC instead.

**Description:**

Currently there is no default push rule for stickers (event type "m.sticker").
This leads to users not being notified about stickers they have received.

Please add default push rules for events of type "m.sticker" to synapse. (maybe add separate rules for direct chats and group chats, just like it is the case for messages)

Alternatively, please allow the default push rules to be extended or overridden by the server admin (like requested in #9325).