Data source for group (from policy document)

[wennergr]

Is your feature request related to a problem? Please describe.
I would like the ability to read the list of members in a group as a data source

Describe the solution you'd like
A new data source that is based on the group key in the policy document

Additional context
I already have a working version in a fork, are you interested in a PR?

[knyar]

I'd probably suggest adding a data resource for the whole policy content (#180) and let the user read whatever part of the policy they are interested in.