Mark iOS push notifications with push_type = alert to allow for on device silencing

[stefanceriu]

We were recently granted the com.apple.developer.usernotifications.filtering entitlement which allows push notifications to be completely discarded once received on the device. This is useful for hiding notifications from encrypted events we don't care about like poll answers (as opposed to showing an empty notification).

One caveat is that it only works for alert apns-push-type notifications which is an optional field we're not currently setting.

This PR does just that through the use of aioapns's PushType header.

[clokep]

What happens if an app isn't approved for this and we set this push type?

[stefanceriu]

Nothing changes as far as I can tell, alert is the correct one for notifications that trigger a user interaction—for example, an alert, badge, or sound.

https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns/

[clokep]

Thanks! Mostly wanted to make sure it wouldn't reject the push completely.

[squahtx]

The apple docs imply that if we use the alert push type, we have to use the non-voip topic:
https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns/

alert
Use the alert push type for notifications that trigger a user interaction—for example, an alert, badge, or sound. If you set this push type, the apns-topic header field must use your app’s bundle ID as the topic. For more information, see Generating a remote notification.

voip
[...] If you set this push type, the apns-topic header field must use your app’s bundle ID with .voip appended to the end.

However it seems that we configure sygnal to use both voip and non-voip topics:

    - name: im.vector.app.ios.prod
      [...]
      topic: im.vector.app

    - name: im.vector.app.ios.dev
      platform: sandbox
      [...]
      topic: im.vector.app

    - name: im.vector.app.ios.voip.prod
      [...]
      topic: im.vector.app.voip

    - name: im.vector.app.ios.voip.dev
      platform: sandbox
      [...]
      topic: im.vector.app.voip

I think it's best that we make a push_type config option that defaults to None.
We could then roll it out to dev builds first, before turning it on for production builds.

[stefanceriu]

This PR is incomplete and would break voip UX. Raised separate ticket here #308