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

**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)](https://github.com/aws-cloudformation/aws-guard-rules-registry/blob/main/rules/aws/elastic_load_balancing_v2/elbv2_acm_certificate_required.guard), 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

Start by reviewing issue #267 and the linked registry example to understand the existing workarounds for retrieving logical IDs. Define what key(this) should return for a node or set of nodes, then verify that the requested example can operate on bucket keys.

Written by the indexing model from the issue text.