Implement MovingAverageOptimizer

[seanpmorgan]

Per the RFC, we need to move MovingAverageOptimizer from contrib to addons:

This will involve inheriting from base Keras optimizer, modifying the code to match those APIs, and modifying test cases to run in TF2.

[Squadrick]

@seanpmorgan Hey, can I implement this?

[seanpmorgan]

@Squadrick Absolutely, thanks for your effort!