fix(terraform): hcl object expressions to return references

[Emyrk]

References included in hcl objects should be returned from 'Attribute.AllReferences()'. Traverse the object fields recursively to locate references.

Expressions such as:

data "foo" "bar" {
  tags = {
    "cacheTTL"   = var.cache
  }
}

Should return variable.cache in the AllReferences()

[CLAassistant]

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