Added missing TF_StringEncodedSize.

[fuszenecker]

Fixes #6893

In the recent versions of TensorFlow, the method TF_StringEncodedSize was removed. I implemented the same code in C#, also added new props for separate SciSharp.TensorFlow.Redist-Linux-GPU and SciSharp.TensorFlow.Redist-Windows-GPU packages.

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[fuszenecker]

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