Introduce OutputBuilder in Java SDK

[kennknowles]

With the addition of extended metadata for elements, we need to make code - especially user code - more robust to added metadata fields. This pull request adds OutputBuilder which should be used to builds new values, and builders should always be obtained by pulling metadata from some context (for example a currently-in-process element) when possible.

Concretely, this pull request

• Adds OutputBuilder as a public interface extending WindowedValue.
• Adds WindowedValues.builder(WindowedValueReceiver) to produce an OutputBuilder that will output to the provider receiver.

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