Modify spec for multi cluster deployment

[ffjlabo]

What this PR does / why we need it:

I changed the location of the configuration for the multi-cluster deployment. Before that, I considered setting the configs on each stage. But it would be better to unify the config for the target setting to reduce duplication.

apiVersion: pipecd.dev/v1beta1
kind: KubernetesApp
spec:
  name: microservice-a
  labels:
    env: prd
    team: product
  quickSync:
    prune: true
  input:
    multiTarget:
      - targetID: hoge
        provider:
          name: cluster-hoge # platform provider name
        manifests: ./cluster-hoge # the resource dir
        kubectlVersion: 1.30
        kustomizeDir: ./cluster-hoge
        kustomizeVersion: v5.4.3
        kustomizeOptions:
          enable-helm: ""
          load-restrictor: "LoadRestrictionsNone"
      - targetID: fuga
        provider:
          name: cluster-fuga # platform provider name
        kubectlVersion: 1.30
        kustomizeDir: ./cluster-fuga
        kustomizeVersion: v5.4.3
        kustomizeOptions:
          enable-helm: ""
          load-restrictor: "LoadRestrictionsNone"

Which issue(s) this PR fixes:

Part Of #5006

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 22.82%. Comparing base (5d3e17d) to head (35ff8ed). Report is 10 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5219 +/- ## ======================================= Coverage 22.82% 22.82% ======================================= Files 420 419 -1 Lines 45302 45297 -5 ======================================= + Hits 10340 10341 +1 + Misses 34166 34161 -5 + Partials 796 795 -1 ```

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