[Go] Support the marking of fields as XML attributes

Created by: langston-barrett

When I mark a property as an attribute, it isn't reflected in the Go code. This:

    T:
      properties:
        Foo:
          type: boolean
          xml:
            attribute: true

should yield

type T struct {
   [...]
   Foo `xml:"Foo,attr"`
}

but currently is missing the attr part.

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