Add support for presence class in Go

[ghost]

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Expected Behavior

Encoding the YDK Go API for below YANG model should produce below output:

    container outer {
        container inner {
            presence true;
        }
    }

<outer>
  <inner/>
<outer>

Current Behavior

presence container is not supported in Go

[ylil93]

the fix can be something like this
https://play.golang.org/

[ylil93]

fixed