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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.