Improve caching: custom CachingLayer and encryption by default

[levochkaa]

We are trying to secure our feature flags, thats why we implemented remote eval and encryption, but then suddenly found, that caching on a client is done without encryption in plain JSON, which can be easily faked.

So I found in the sources that the SDK already has CachingLayer protocol, but for some reason it wasn't public. If it was I would probably just implement custom caching mechanism with encryption.
After reading the code (for the 232 time lol), I thought that implementing encrypted cache is pretty easy, so here is the PR, which proposes two changes:

1. A way to set a custom CachingManager/Layer
2. Encrypted cache if encryptionKey is provided

This change will result in a Failed get features from cached encrypted features for the first start after update for an old user, but it seems ok for me.