feat: added a function to compute population coefficient between target year and latest available year from world bank

[JinIgarashi]

fixes #146

added the below function to compute coefficient.

def get_pop_coefficient_world_bank(target_year: int, country_code: str) -> list[int | float]:
    """
    Fetches population data from the World Bank API and calculates the population growth coefficient.
    Parameters:
        target_year (int): The base year for comparison.
        country_code (str): The ISO3 country code.
    Returns:
        list[int, float]: A list containing the latest available year and the calculated coefficient.
    """

this is called at parent function of get_pop_coefficient and will compare both result of world bank and UNSD to get latest one. currently UNSD is not supported.

Also, added some simple test cases by mocking world bank api.