allow registering custom layers in prune and clustering registries.

[aaarrti]

Hi team tensorflow-model-optimization, I was applying clustering and pruning to a model based on Bert encoder from tensorlfow-models-official, and noticed that there is no API for registering custom layers, so here is a PR, which adds it.

Implemented changes

• Allow registering custom layers in PruneRegistry and ClusteringRegistry
• Export PruneRegistry and ClusteringRegistry as part of public API.