Add example for using CachingMostRecentProvider and upgrading material version

### Problem:

The AWS docs mention "rotating cryptographic material" here: https://docs.aws.amazon.com/dynamodb-encryption-client/latest/devguide/most-recent-provider.html

However it could be more clear what "call the provider store's Create New Provider operation with the material name of the Most Recent Provider" means for this library.

### Solution:

This repo should have an example showing how to do this using the Metastore's newProvider() method.

### Out of scope:

Updating the corresponding AWS docs to reference this new example.