[feature] Make Persistence Agent use SA Token when calling KF APIServer endpoints

[difince]

Feature Area

</area backend>

What feature would you like to see?

Currently, when the Persistence Agent (PA) calls KFPipeline APIs - readArtifacts and ReportMetrics - It authenticates as a user.
See:

pipelines/backend/src/agent/persistence/client/pipeline_client.go

Lines 196 to 204 in 110e082

	pctx := context.Background()
	if user != "" {
	pctx = metadata.AppendToOutgoingContext(pctx, getKubeflowUserIDHeader(),
	getKubeflowUserIDPrefix()+user)
	}
	ctx, cancel := context.WithTimeout(pctx, time.Minute)
	defer cancel()
	response, err := p.runServiceClient.ReportRunMetricsV1(ctx, request)

The proper way for service-to-service authentication/authorization is the use of a Service Account Token. PA SA token has been already introduced by this PR .
My suggestion is to use the PA SA token for readArtifacts and ReportMetrics as well.
Thus the communication between PA and KFPipeline Api service will be done in the correct way. The code will become clearer/shorter and the unnecessary requests to the kubeapi will be removed.

What is the use case or pain point?

Is there a workaround currently?

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[difince]

/assign @difince