[Plan Preview] Enable to compare running resources and target manifests

[ffjlabo]

What would you like to be added:

We want to support the plan preview with the diff between actual resources and target manifests, discussed in https://github.com/pipe-cd/pipecd/pull/5986

At first, we won't pass RunningDeploySource because it's better to compare TargetDeploySource and LiveState. In the future, we might add RunningDeploySource to GetPlanPreviewRequest or add a new rpc.

Why is this needed:

For users, it would be nice to know what happens after deploying.