Added event handlers and tests

[calum-mcg]

Summary: Added optional custom event handlers for new_key and existing_key. Also added tests to cover the event handlers and custom key builder

Example For the events new_key and existing_key you can pass in a custom handler. By default there are no handlers.

Via a decorator:

@FastAPICache.on_event("existing_key")
def exists_in_cache(func, *args, **kwargs):
    print("Existing key")
    return None
@FastAPICache.on_event("new_key")
def new_in_cache(func, *args, **kwargs):
    print("New key set")
    return None

Via a class method:

def exists_in_cache(func, *args, **kwargs):
    print("Existing key")
    return None
def new_in_cache(func, *args, **kwargs):
    print("New key set")
    return None
FastAPICache.set_on_existing_key(exists_in_cache)
FastAPICache.set_on_new_key(new_in_cache)

[calum-mcg]

@long2ice - I've updated the PR based on your comments. Could you merge?

[calum-mcg]

@long2ice - any updates? Let me know if there are any other changes required

[long2ice]

OK, please resolve conflicting files

[calum-mcg]

Done!

[long2ice]

CI not pass, please merge master and try again

[mustyoshi]

Any movement on this? Would like to build off of it for some additional contributions.

[calum-mcg]

@long2ice - needs to run and pass CI workflow then can merge

[calum-mcg]

@long2ice - any updates on this?