feat: 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

• notification_center_registry_spec.rb
• project_spec.rb
• http_project_config_manager_spec.rb
• optimizely_factory_spec.rb

Ticket

FSSDK-8775