attach handler to engine via multiple user-specified events

[wyli]

Is your feature request related to a problem? Please describe.
currently, the handler's attach method such as

MONAI/monai/handlers/checkpoint_saver.py

Line 220 in fa5bc15

def attach(self, engine: Engine) -> None:

deals with a fixed set of events

there are a few possible solutions but we'd like to get more feedback first before changing the code