Add support for async dropScope function

[akagr]

• I'm submitting a ...
  [ ] bug report
  [x] feature request
  [ ] other

• What is the current behavior?
  Right now, the dropScope has to be defined synchronously. If it's a function, it has to return true or false.

• What is the expected behavior?
  I propose we allow the dropScope to be set to a function that returns an observable. That observable may then resolve to true or false. This would allow for scenarios, where say, we need to check with backend if the drop is allowed via http calls.

I'm willing to give developing this a try myself and PR it if it sounds good.

[ObaidUrRehman]

@akagr LGTM, the dropScope is already defined as string | Array<string> | Function so adding an Observable shouldn't be an issue.

Looking forward to your PR.
👍

[akagr]

@ObaidUrRehman Great thanks! Will try to get to it this weekend 😄

[ObaidUrRehman]

Added in v3.0.1