Add `current` methods for shop and recurring application charges

[paulomarg]

Description

Fixes #923

As pointed out, we'd removed some utility methods from the Shop and RecurringApplicationCharge resources, which created problems for apps that relied on those methods.

This PR adds back those methods.

How has this been tested?

With unit tests, and by calling the methods in an app using the library.

Checklist:

• [x] My commit message follow the pattern described in here
• [x] I have performed a self-review of my own code.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added a changelog line.

[kirillplatonov]

This is awesome, thank you! You even re-created exact same API 👏