feat: add cover image and avatar decoration

[SuperZombi]

Also fixed invalid syntax. Sorry

    discordrpc\presence.py", line 314
    def _send(self, payload, op=OP_FRAME: int):
                                        ^
SyntaxError: invalid syntax

[SuperZombi]

Needs to add to DOCS:
User.avatar_decoration and App.cover

[Senophyx]

Merged. Thank you.