Support TargetName binding of nested properties

[BernieWhite]

When an object is passed from the pipeline, PSRule assigns the object a TargetName. TargetName is used in output results to identify one object from another.

Currently direct properties of the pipeline object can be bound, or a custom script block can be used to perform binding.

Using a custom script block is highly flexible but has performance impacts. For simple cases like nested properties, it would be better to provide this feature natively.

A similar feature was added in #60 for Exists, Within and Match keywords.

For example:

# Use the metadata.name property as TargetName
binding:
  targetName:
  - 'metadata.name'

An example Kubernetes namespace object:

---
# Namespace: dev
apiVersion: v1
kind: Namespace
metadata:
  name: dev

Using metadata.name to bind TargetName should return dev.