[3.2] [iOS] Pluggable application delegate

[naithar]

Same as #43556 for 3.2 branch.
Also depends on #41340 being merged first.

Push notification example

Plugin repo: https://github.com/naithar/godot_ios_plugin_apns

Usage:

func _ready():
	print("start")
	if Engine.has_singleton("APNS"):
		print("connecting apns")
		apns = Engine.get_singleton("APNS")
		apns.connect("device_address_changed", self, "_on_device_address")
		apns.register_push_notifications()
	
func _on_device_address(device_address):
	print(device_address)

Result:

[akien-mga]

Thanks!