Upload pipeline version return a response with enum as string

[jingzhang36]

Our api client assumes the enum being string e.g. https://github.com/kubeflow/pipelines/blob/master/backend/api/go_http_client/pipeline_upload_model/api_resource_type.go
So when we populate the response, we need to treat enum as string instead of int, in order for our api client to parse it correctly.

Needed to unblock another PR at #3174

Tested on my own KFP instance with latest image at https://pantheon.corp.google.com/kubernetes/deployment/us-central1-a/kubeflow-pipelines-standalone/kubeflow/ml-pipeline?project=jingzhangjz-project&organizationId=433637338589&tab=overview&deployment_overview_active_revisions_tablesize=50&duration=PT1H&pod_summary_list_tablesize=20&service_list_datatablesize=20

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This change is 

[Bobgy]

/lgtm
/approve

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