Make WindowedValue a public interface, and receivers for it

[kennknowles]

This is a preliminary change to reduce the number of files to review in #34902

When we introduce OutputBuilder, it will implement WindowedValue, and every time we create an OutputBuilder we will want to provide

1. the default metadata for the output
2. the receiver for the output

This change unblocks step 2 and reduces the size of the final change. The exact things done in this PR cause a lot of tiny changes but they are trivial:

1. Move WindowedValue out of internal util package and into public values.
2. Make WindowedValue an interface (it was an abstract class for obsolete reasons for Java < 8, and we want other interfaces to be able to extend it)
3. WindowedValues companion class for concrete implementations and static methods (that already exist).
4. Add WindowedValueReceiver and WindowedValueMultiReceiver in the SDK (previously they were in runners/core) and use them. This simplifies and eliminates a decent chunk of runner code.

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[github-actions]

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[kennknowles]

Actually I think I can make it even simpler and one huge totally automated change that doesn't need much review and the semi-useful changes in a later PR.