Refactor 'destination' configuration for consistency across Backup and Application

[Xieql]

What would you like to be added:
Refactor 'destination' configuration for consistency across Backup and Application

Why is this needed:
In our current implementation, we have a destination configuration that behaves differently in Backup and Application. In Backup, it allows specifying a cluster, while in scenario Application, it allows targeting a group of clusters with a specific label.

This discrepancy can lead to user confusion as the goal is the same—to designate a target destination cluster for a task—but the method differs.

To address this, I propose we refactor the 'destination' configuration to support both settings methods—specifying a single cluster or a group of clusters with a tag—in both Backup and Application.

[hzxuzhonghu]

What do you think and how we can better design the target clusters

[Xieql]

What do you think and how we can better design the target clusters

I think we can support both settings methods in any crd