Refactor packages and separate internal and pkg

[powerfooI]

Summary

1. Move auxiliary type definitions from api/v1alpha1 to api/types like BackupVolumeSpec.
2. Move task-related types into pkg/task like OperationContext.
3. Move controller, resource, telemetry and const folder in previous pkg/ to a new folder internal/.
4. In resource folder, the PR separate every resource manager into single folder, in which names of their tasks and task flows are included. This change increases locality of code of every resource manager and make pkg/task a quite independent package that does not rely on ob-operator actually. Every resource manager register their task flows in their own package init part.
5. Move struct coordinator and interface ResourceManager to a new package coordinator in pkg/ folder, which is another independent module, too.
6. In order to avoid possible race when registering task flows, a simple mutex was added to task_manager. I believe that it does not impact the functionality of task registry. This is the only change that impacts functionalities in this PR.
7. Set specific types to fields (TaskName, FlowName, TaskID etc.) in pkg/task instead of plain string that may cause confusion in some cases. (e.g. mismatch name of task flows and tasks).

This PR may somehow reach the requirement of #128

[chris-sun-star]

LGTM