Implement @documented decorator for functions

[anatoly-scherbakov]

@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.