🚀 Enhance support for variable sets and refactor the implementation

[sheneska]

Description

This PR goes over some code refactoring for the variable sets support and add the ability to refer to it by name.

Tests

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Usage Example

apiVersion: app.terraform.io/v1alpha2
kind: Workspace
metadata:
  name: this-vs
spec:
  organization: kubernetes-operator
  token:
    secretKeyRef:
      name: tfc-operator
      key: token
  name: kubernetes-operator
  variableSets:
  - name: name

References

N/A.

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