Add Support for Scheduler API

[Savant-Dev]

This API should be only be accessed through bot/__init__.py

Needed Plugin Functions:

• scheduling tasks api/scheduler.py : API.schedule()
• executing tasks bot/__init__.py : DiscordBot.task_executor() abstract
• canceling tasks api/scheduler.py : API.cancel() & API.cancel_all()
• job retrieval api/scheduler.py : API.view_jobs() & API.find_job()
• scheduler control api/scheduler.py : API.shutdown_executor() & API.start_executor()

Preferably done with as little impact to bot/__init__.py as possible - Low Footprint