Encrypt Token

[alejcas]

We should by default (but configurable) encrypt and decrypt the token info.

This will be a task done by the TokenBackend chosen.
The default BaseTokenBackend should implement an option to encrypt that will be active by default.

The option can be passed from the Account to the Connection to finally end at the TokenBackend.

The key to decrypt and encrypt should be configurable but with a default (maybe the client_secret can be the default key to the encryption?

This will maybe add another requirement to this library.
Maybe this should be only an example of a Custom BaseTokenBackend like a SecureBaseTokenBackend but I feel kind of obliget to include this as a default.

[Ashesh3]

+1 looking to encrypt the token file somehow