Add WL task resource and use it in the client

[serhii-karelov]

Summary

In the scope of in-app purchases implementation for Bottomline, we need to extend yolapy to query new wl-task endpoints.

Todo

• add WLTaskResourceMixin (see exisiting mixing from yolapy/resources/*.py for inspiration)
  • the mixin should have methods to query wl-task endpoints (follow the link).
• use the new mixin in the HTTP client.