Add key rotation support

[KarlGallagher]

As per title. I cannot see this defined in the features list, so asking for clarification purposes.

[joeyparrish]

Not yet, but it is supported by Shaka Packager with Widevine, so it should be easy to add. https://google.github.io/shaka-packager/html/tutorials/widevine.html

If you are interested to create a PR for this, let me offer you a few pointers:

• The pipeline_configuration module, where a config for crypto period duration could be added in EncryptionConfig: https://github.com/google/shaka-streamer/blob/master/streamer/pipeline_configuration.py
• The packager_node module, where that config can be used to add the --crypto_period_duration option to the Shaka Packager command line: https://github.com/google/shaka-streamer/blob/master/streamer/packager_node.py