feat(object_to_json): add new exercise

subjects/devops/object_to_json/README.md

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+## object_to_json
+
+A lot of information on the web are shared in the JSON format. This exercise will be about transforming object to JSON and vice-versa.
+
+### Instructions
+
+You just landed a new job, congrats! Your new task is to build two functions to allow new users to register to your new shiny website.
+
+The new registration information comes as string formatted as JSON. You need to create a file `registration.py` that will have the following functions inside:
+
+1. `create_new_user` that will receive a `dict` and will return a new object of the class `User` provided below. To be valid, the input `dict` must have a `username` key and a `email` key to be valid. The new `User` will have the same `username` and `email` of the input `dict`. If the input `dict` is invalid, the default user will be returned.
+
+```python
+class User:
+    username = 'user'
+    email = 'something@email.com'
+```
+
+2. `user_to_json` that will receive a `User` and will return the object as a string in JSON format. Be aware of the Python types that can be converted to JSON!
+
+### Usage
+
+Here is a possible `test.py` to test your functions:
+
+```python
+import create_new_user
+import user_to_json
+
+registration_0 = '{"username": "mario", "email": "mario@me.it"}'
+registration_1 = '{"city": "Rome", "country": "Italy"}'
+
+user_0 = create_new_user(registration_0)
+user_1 = create_new_user(registration_1)
+
+print(user_to_json(user_0))
+print(user_to_json(user_1))
+```
+
+```console
+$ python3 test.py
+{"username": "mario", "email": "mario@me.it"}
+{}
+$
+```
+
+### Hints
+
+- It is possible to convert an object to `dict` using the `__dict__` casting.
+
+```python
+class C:
+    a = 1
+    b = 2
+
+my_obj = C()
+my_obj_dict = my_obj.__dict__ # my_obj_dict will be a dictionary with the object my_obj values
+```
+
+### Notions
+
+- [Class and Object](https://docs.python.org/3/tutorial/classes.html#a-first-look-at-classes)
+- [json.loads()](https://www.geeksforgeeks.org/json-loads-in-python/)
+- [json.dumps()](https://www.geeksforgeeks.org/json-dumps-in-python/)