Transfer events and notifications service refs from their plugins to the backend-plugin-api package

[camilaibs]

Context

Ref: #24493

Backend service references are inconsistently provided in core and plugin packages. We intend to use the permissions model, where the service reference resides in the backend-plugin-api, but the default service is implemented in the plugin package.

Description

Similar to permissions, the "events" and "notifications" service references should be extracted to the "coreService" namespace in backend-plugin-api.

Note

There is a related pull request opened, please refer to avoid conflicts.

Acceptance criteria

• Extracted code should be re-exported from the new module in the old package
• Deprecation exports in the old packages must be declared in a deprecated.ts file
• Deprecation exports should be annotated with @deprecated and contain a comment explaining their replacements
• Other packages' import statements should be updated to use bindings from the new package
• Documentation examples should be revisited to use code from the new package instead
• The backend-test-utils package should provide a mock and a factory for all core services (e.g. see)

[camilaibs]

Events PR: #24912

[camilaibs]

Notifications PR: #24917

[camilaibs]

We decided to push back on this and give the events and notifications services more time to be broadly used before moving it into core services.
cc/ @Rugvip