Add feature: call with freshness threshold

[Daniel-Chin]

The feature

In addition to specifying shelf life at storage time, this supports specifying max age at retrieval time.

Motivations

This is useful in interactive projects / data science notebooks.
I personally came to need this when building a cachable API client for Semantic Scholar. (Who knows when's the next groundbreaking paper? I only know how much cache to trust in hindsight.)

How to use

Refer to the test:

def test_call_with_freshness_threshold():
    @cachier.cachier()
    def test_func(a, b):
        print('Computing...')
        return a + b
    
    print(f'{test_func(1, 2) = }')
    print(f'{test_func(1, 2) = }')
    caller_with_freshness_threshold = test_func.caller_with_freshness_threshold(
        timedelta(seconds=0.5), 
    )
    print(f'{caller_with_freshness_threshold(1, 2) = }')
    print(f'{time.sleep(1.0) = }')
    print(f'{test_func(1, 2) = }')
    print(f'{caller_with_freshness_threshold(1, 2) = }')

Anecdote

        def _caller_with_freshness_threshold(max_age: timedelta):
            return wraps(func)(partial(_call, max_age))
        
        func_wrapper.caller_with_freshness_threshold = _caller_with_freshness_threshold

can in fact be further reduced to

func_wrapper.caller_with_freshness_threshold = partial(partial, _call)

(how fun!) but then how do you apply wraps(func) to type hint it? I haven't the faintest.

[shaypal5]

Hey Daniel, :)

A very cool feature! Thank you for contributing.
I approved tests to run for your PR, and it seems you broke ignoring of self in certain cases.

Please try and solve this. And tag me or @Borda if you need help.

Cheers!
@Daniel-Chin