CMM cache policy on encryption and decryption DEKs

### Problem:

While profiling an application I could find latency spikes, relate to decryption

If an application is regularly using a DEK for decryption, it can expire due to its TTL
As I read the code the TTL is set when the key is created. Is seems sensible that the key would expire when not used, but if used frequently why should it expire?

as [ajewellamz](https://github.com/ajewellamz) poned our when I raised this in the wrong project -https://github.com/aws/aws-encryption-sdk/issues/841 https://github.com/aws/aws-encryption-sdk/issues/841#issuecomment-3628451811 it is also experied to ensure correctness, and that the application has access to decrypt

We may have several thousand decryption DEKs in the cache, and regularly in use, and then we see a spike of a many decryption DEKs being regenerated, because they have expired due to TTL, which causes application latency (and some cost)

We have implemented a mechanism to rotate encryption DEKs as we know the limited set of keys in use. Effectively just regenerate the key 10 second before it would expire, but his path doesn't block encryption calls as it doesn't evict from the cache, it just replaces the entry when regenerated - https://github.com/aws/aws-encryption-sdk/issues/840

### Solution:

I think there could be some option to refresh the DEKs before they expire, to keep DEKs that are in use (within some time window), without DEK access causing latency to the calling app

We have some code in our project that does this for the encryption DEKs, which we could export to this library if its useful to others, which I imagine it would be

### Out of scope:

Start by reviewing the CMM cache policy and the linked AWS Encryption SDK issues 840 and 841 to understand the existing DEK refresh and expiration behavior. Define what configurable refresh behavior should apply to frequently used decryption DEKs, including its interaction with TTL and cache replacement; done means the policy is specified and its latency and correctness effects are covered.

Written by the indexing model from the issue text.