@Override
    public boolean equals(Object obj) {
        if (!(obj instanceof GetFieldExpression rhs)) return false;
        if (!structFieldName.equals(rhs.structFieldName)) return false;
        return true;
    }
Returns the value of a struct field.

@author Simon Thoresen Hult
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/GetFieldExpression.java