Introduce WindowedValue receivers and consolidate runner code to them

[kennknowles]

We need a receiver for WindowedValue to create an OutputBuilder. This change introduces the interface and uses it in many places where it is appropriate, replacing and simplifying internal runner code.

This is another piece pulled out of #34902 and revised to be standalone and more carefully reviewed.

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[kennknowles]

R: @stankiewicz

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[stankiewicz]

thanks, it nicely simplifies how doFn will output WindowedValues

[kennknowles]

Since Dataflow tests are expensive, I am noting that they passed. I will push a fix for spotless.