Add `chef_vault()` helper method to efficiently list items in a vault

[saidmasoud]

Description

This PR adds a helper method to list items in a vault. Per the below referenced issue, data_bag() was suggested to be used to list items in a vault. However, that method also returns the keys in addition to the items, forcing users to parse the list and remove the keys. The new method essentially does this for users to keep their code bases cleaner.

Issues Resolved

66

Check List

• [Y] All tests pass. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/TESTING.MD
• [Y] New functionality includes testing.
• [Y] New functionality has been documented in the README if applicable
• [Y] All commits have been signed for the Developer Certificate of Origin. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/CONTRIBUTING.MD

[saidmasoud]

Thanks for the comments, @thommay! I have addressed them with my latest commit.

[thommay]

Awesome, thanks!