Lab Encrypted Storage: Allow customizing the serialization/deserialization implementation

[harri35]

As the integrator of Lab Encrypted Storage library,

I want to be able to customize the DTO serialization/deserialization implementation,

so I have the option to create one that is best for me.

Criteria:

• Create a way and guidance to customize serialization/deserialization
• Refactor the current Gson one to match this interface
• Make the current one default
• Allow setting a custom one
• Create tests as needed