feat(backend): enforce SA Token based auth b/w Persistence Agent and Pipeline API Server

[difince]

Fixes: #9937

Enforce Service Account(SA) Token-based auth between the Persistence Agent and the Pipeline API Server for all the requests.
SA Token auth has already been introduced and applied for ReportWorkflow and ReportScheduledWorkflow by this PR.

Current PR enforces the SA token authentication for readArtifacts and ReportMetrics calls as well, which currently authenticates as a user.

Description of your changes:

This PR :

• reverts changes done by this PR
• Enforce the use the SA token authentiaction

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[difince]

/retest

[difince]

Can someone help me with the failing test - TestCreatePipelineV1_LargeFile ?

[difince]

To simplify the review process, the changes in pipelines_client.go ReadArtifact and ReportRunMetrics are the most meaningful ones. Here, the PA is forced to use SA Token, instead of authenticating as a user.

[chensun]

Can someone help me with the failing test - TestCreatePipelineV1_LargeFile ?

Seems like it could be some transient issue?

    pipeline_server_test.go:99: 
        	Error Trace:	/home/prow/go/src/github.com/kubeflow/pipelines/backend/src/apiserver/server/pipeline_server_test.go:99
        	Error:      	Expected nil, but got: &util.UserError{internalError:(*errors.withStack)(0xc00127e090), externalMessage:"error fetching pipeline spec from https://raw.githubusercontent.com/kubeflow/pipelines/master/sdk/python/test_data/pipelines/xgboost_sample_pipeline.yaml - request returned 503 Service Unavailable", externalStatusCode:0x3}
        	Test:       	TestCreatePipelineV1_LargeFile
    pipeline_server_test.go:100: 

[chensun]

/retest

[chensun]

/lgtm
/approve

Thanks!

[google-oss-prow]

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