Add Support for mutable_content in aioapns

[HashimJVZ]

Re-do of #745

Changes in the #745 was 314a484

This PR introduces support for the mutable_content property in the aioapns library, which was previously available only in apns2. The mutable_content property allows notifications to be modified by a Notification Service Extension before being delivered to the user.

This is related to #320 and #350.
Earlier support was available in the apns.py file, now added to apns_async.py.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.50%. Comparing base (44977b1) to head (e0e098c).
Report is 52 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #755      +/-   ##
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+ Coverage   70.39%   72.50%   +2.10%     
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  Files          27       29       +2     
  Lines        1101     1240     +139     
  Branches      180      205      +25     
==========================================
+ Hits          775      899     +124     
- Misses        288      298      +10     
- Partials       38       43       +5     

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[50-Course]

Hi @HashimJVZ,

This is great, I had some review in the past four days -- however, i couldn't fully complete it due to one or two. Going through this again, I have one or two questions -- so we can call this in 😄 .

Foremost, this looks great!

django-push-notifications/tests/test_apns_async_push_payload.py

Lines 185 to 208 in e0e098c

	def test_push_payload_with_mutable_content(self, mock_apns):
	apns_send_message(
	"123",
	"Hello world",
	mutable_content=True,
	creds=TokenCredentials(key="aaa", key_id="bbb", team_id="ccc"),
	sound="chime",
	extra={"custom_data": 12345},
	expiration=int(time.time()) + 3,
	)
	args, kwargs = mock_apns.return_value.send_notification.call_args
	req = args[0]
	# Assertions
	self.assertTrue("mutable-content" in req.message["aps"])
	self.assertEqual(req.message["aps"]["mutable-content"], 1) # APNs expects 1 for True
	# def test_bad_priority(self):
	# with mock.patch("apns2.credentials.init_context"):
	# with mock.patch("apns2.client.APNsClient.connect"):
	# with mock.patch("apns2.client.APNsClient.send_notification") as s:
	# self.assertRaises(APNSUnsupportedPriority, _apns_send, "123",
	# "_" * 2049, priority=24)

however, we are introducing mutable_content as int (rightfully so) in this change, as opposed to previous versions built upon apns2 library (mutable_content as boolean - which does the handling underneath, constraining the top-level API to boolean values), i think the former anti-pattern with the boolean values is better here, as it avoid the unnecessary shenanigans with magic numbers from user inputs. However, we with good doc, we good. Can we document this new change?

It would help smooth the smooth transition in the release note. Other than that, this looks ready for join the mainline 👍🏼