fix: add notification center registry

[andrewleap-optimizely]

Summary

Create a private registry of NotificationCenters for internal use.

• Remove dependence on user supplied NotificationCenter.
• Resolve issue of notifications being registered with one NotificationCenter and sent to a different one.

Test plan

added tests to

• test_notification_center_registry.py
• test_optimizely.py
• test_optimizely_config.py
• test_optimizely_factory.py

Ticket

FSSDK-8773

[jaeopt]

@andrewleap-optimizely can we keep a note about this breaking change. We should release a major version bump with this note.