feat(backend): Add TTL Strategy in Workflow Spec for Post-Completion Cleanup

[VaniHaripriya]

Description of your changes:

• Added the TTLStrategy field in Workflow Spec, configured to use SecondsAfterCompletion.
• Uses the pipeline_ttlseconds variable to define the TTL duration.
• This field automates cleanup of completed workflows.

This PR should be merged only after #11269 gets merged.

Testing Instruxtions:

• Build the API Server image and push to an image registry
• Upload main.yaml file from here
• Check in KFP UI Pipeline Spec tab if the following snippet is present:

platform_spec:
  platforms:
    kubernetes:
      pipelineConfig:
        pipelineTtl: 60

• Create a run and check if TTL configuration is present in Workflow CR:

$ oc get workflow -o yaml $(oc get workflow --no-headers | awk '{print $1}') | yq .spec.ttlStrategy
secondsAfterCompletion: 60

Checklist:

• You have signed off your commits
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[google-oss-prow]

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