Added `from_hex_string` which gets us `EncryptionKey` from a
hex string. Now we can use securely generated keys, like from openssl
$ openssl rand -hex 32
Let's add an encryption module, hard coded to use AES 256 GCM.
Other required parameters are also hard coded and will be made
configurable in the future PRs.
The module is behind a `encryption` feature flag.
