Refactor: base entity classes

[nadavelkabets]

Part of #1620

Changes

• New BasePublisher, BaseService, BaseSubscription, BaseClient and BaseTimer classes
• __init__ parameter changes
  • Subscription, Publisher: callback_group and event_callbacks are now keyword only
  • Service, Client, Timer: callback_group is now keyword only
  • Timer: callback_group defaults to None

Notes

• Callback groups and event handlers were not moved to base classes
  • Callback groups are exclusive to executors and would not be supported in AsyncNode. More detailed discussion is in Feature: new asyncio executor #1399.
  • We have no callback mechanism for waitables yet, so AsyncNode won't support event handlers for now.
• Service's send_response() is used publicly only in executor, so it was made protected in the BaseService

[mjcarroll]

• Linux
• Linux-aarch64
• Linux-rhel
• Windows