Add 3 examples in playground SQL transform, schema transform and Composite Combine

[razvanculea]

3 simple examples of applying multiple combiners on the input PCollection (Min,Max,Sum).

3 different ways of achieving this in Beam

• SQL transform, using the powerful SQL syntax
• Schema transformations, leveraging schemas and Group.byFilelds
• Composed Combiners, same results without the need to use Rows or Schema transforms and with full control over the combiner logic

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[razvanculea]

@damondouglas this blog post will explain how to apply multiple combiners to an input PColl in different ways with Beam and link to these 3 examples