Implement @documented decorator for functions

```python
@documented
def calculate_customers_report(customers: List[Customer], requested_by: User) -> CustomerReport:
"""
Build customers report.

Requested by: {requested_by}.
"""
```

When this function is executed, the following message can be printed:

```
Build customers report.

Requested by: John Doe.
```

Before executing the function, its wrapper will substitute the arguments of the function call into the docstring and then will print the docstring using built-in Python logging system.

Purpose of this is to illustrate what is happening in the program in a human readable way in its log.