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:

• [x] You have signed off your commits
• [x] The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow[bot]]

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