Crypto Materials Cache entry identifier vectors

We need these. Currently, we just have some duplicated in the test codebases. We should pull them out into a manifest spec/instance.

https://github.com/aws/aws-encryption-sdk-python/blob/master/test/unit/test_caches.py

Start by reviewing the duplicated crypto materials cache entry identifiers in test/unit/test_caches.py from the linked aws-encryption-sdk-python repository. Define the manifest spec/instance needed to hold these vectors and confirm that the duplicated test data can be represented there; the issue is done when the requested cache entry identifier vectors are available through that manifest.

Written by the indexing model from the issue text.