[Enhancement] Add support to retrieve the key of a given node

[joshfried-aws]

Is your feature request related to a problem? Please describe.

Currently users are unable to retrieve the key of a given node. This is especially troublesome when working with cfn templates, and users want to operate on logical id(s). Currently there are some workarounds to achieve this (see this example from the registry), but it doesnt support all use cases.

Describe the solution you'd like

A function that provides the key of for a node(s). ex

let buckets = Resources.*[ type == "AWS::S3::Bucket" ]

%buckets {
  let bucket_key = key(this) 
  # do something with key
}

Additional context
#267

[ysdholak]

We are facing the same issue. Having this functionality will def help!