feat: allow to send the headers parameters to pywebpush

[aleaxis09]

• Version of pywebpush updated using >= (Latest versions of pywebpush receive headers parameters).
• Added headers parameter to the django-webpush functions, empty dict by default.
• For apple web push notifications some headers are added by default.
• Documentation updated.

[safwanrahman]

Thanks for the PR. Can you make the changes for mutable default value so we can merge it?

[josephetareh]

any updates on merging this?