Introduce fromJson method

## Steps to reproduce:
1. Take a look at the methods inside `BunqModel`

## What should happen:
1. There is a method to convert a JSON to class defined like `to_json`

## What happens:
1. :point_up_2: is not implemented 

## Logs
- Logs `no logs`

## Extra info:
- Tested on [0.12.2](https://github.com/bunq/sdk_java/releases/tag/0.12.3)

## References 
- bunq/sdk_php#84