Allow injection of custom cipher

[albertodebortoli]

In version 0.3.0, ToggleManager accepts an optional CipherConfiguration used to encrypt/decrypt values.
The only supported cipher is ChaCha20Poly1305.

@dchakarov suggested to move ChaCha20Poly1305 out of the package and to leave the implementation of it and/or other Ciphering objects to the integrator.

While I see his point of view, I want to leave it as it is at the moment so to provide a simple solution out-of-the-box.

Would be nice though if the library allowed the injection of a custom cipher.