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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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[khanhtc1202]

Good point specified 👍

[Warashi]

👍🏻