Matrix -> WhatsApp sync user profile pictures

[Sir-Photch]

I suggest we add synchronization of matrix-avatar and whatsapp-avatar, that means: changes to the matrix-user's avatar will be synced to whatsapp. (Optionally behind a config flag.)

Setting the whatsapp avatar is possible according to https://github.com/tulir/whatsmeow/issues/325#issuecomment-1407486249