Avoid length-prefix-bytes substitutions for Flink boundaries.

[robertwb]

The runner infrastructure typically substitutes "unknown" coders for length-prefixed bytes, but for elements going into or out of the Beam pipeline fragment we want the actual objects. Fortunately, we know these coders are known on the Runner side as well as the SDK side as we have them in hand there.

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[chamikaramj]

Ready for review ?

[robertwb]

Yes, this is ready for review. Would you be willing to take a look.

[robertwb]

R: @kennknowles

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