Loads and validates encryption keys from application config.

Expected config

config :kryptex,
  keys: [
    %{id: 1, key: System.fetch_env!("KRYPTEX_DEK_1")},
    %{id: 2, key: System.fetch_env!("KRYPTEX_DEK_2")}
  ],
  default_key_id: 2

Keys may be provided as raw 32-byte binaries or base64 strings.