[push notification plugin] init + WIP android part

[flapili]

TODO:

• [ ] define spec
• [ ] documentation / Readme / etc
• [ ] IOS
  • [ ] topic subscription
  • [ ] topic unsub
  • [ ] refresh token
  • [ ] get Token
  • [ ] set/get auto init
  • [ ] get last notification data
• [ ] Android
  • [x] Topic subscription
  • [ ] topic unsub
  • [ ] refresh token
  • [ ] set/get auto init
  • [ ] get last notification data
• [ ] js guest
  • [x] Topic subscription
  • [ ] topic unsub
  • [x] get Token
  • [ ] refresh token
  • [ ] set/get auto init
  • [ ] get last notification data
• [ ] ensure the config is filled to avoid crash at runtime