For this we need a priority object in the "APNS" object of the sender payload. the format is discussed here:
https://aerogear.org/docs/specs/aerogear-unifiedpush-rest/index.html#397083935

Relevant UPS code:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/1.1.x-dev/push/sender/src/main/java/org/jboss/aerogear/unifiedpush/message/sender/apns/AeroGearApnsPushNotification.java

we need to use a different ctor, from its superclazz:
https://github.com/relayrides/pushy/blob/d2b916191ebcef3ca80d5c5235a3ee13c8cc7a2e/pushy/src/main/java/com/relayrides/pushy/apns/util/SimpleApnsPushNotification.java#L88

The Apple doc on it's "apns-priority" is here:
https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html