[Multi_K8s-Plugin] Add per-stage multiTarget filtering

[mohammedfirdouss]

What this PR does / Why we need it:

Allows each pipeline stage in the kubernetes_multicluster plugin to declare which deploy targets it applies to via a multiTargets list in the stage options. If the list is empty the stage applies to all targets as before, so existing configs are unaffected.

Without this, every stage always runs on all targets. Users cannot do progressive multi-cluster delivery like canary to one cluster first, wait for approval, then roll out to others.

With this change:

pipeline:
  stages:
    - name: K8S_MULTI_CANARY_ROLLOUT
      with:
        replicas: 10%
        multiTargets:
          - cluster-us
    - name: K8S_MULTI_WAIT_APPROVAL
    - name: K8S_MULTI_CANARY_ROLLOUT
      with:
        replicas: 10%
        multiTargets:
          - cluster-eu
    - name: K8S_MULTI_PRIMARY_ROLLOUT

Which issue(s) this PR fixes:

Part of #6446

How was this tested:

• Added TestFilterStageTargets to deployment/misc_test.go with 5 cases: empty filter returns all targets, single target selected, multiple targets selected, unknown target name silently ignored, mix of known and unknown names.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.52%. Comparing base (e37df69) to head (843c84f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6757      +/-   ##
==========================================
+ Coverage   29.33%   37.52%   +8.19%     
==========================================
  Files         598       94     -504     
  Lines       63902     7677   -56225     
==========================================
- Hits        18744     2881   -15863     
+ Misses      43711     4651   -39060     
+ Partials     1447      145    -1302     

Flag	Coverage Δ
.	?
.-pkg-app-pipedv1-plugin-analysis	32.43% <ø> (ø)
.-pkg-app-pipedv1-plugin-ecs	31.82% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	?
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	?
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	37.95% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	33.04% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	52.71% <ø> (ø)
.-pkg-plugin-sdk	50.34% <ø> (ø)
.-tool-actions-gh-release	?
.-tool-actions-plan-preview	25.51% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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