Fairing tries to list secrets instead of get when it just need to check for existence of a single secret

[Ark-kun]

The secret_exists function is incorrectly using [list_namespaced_secret](https://github.com/kubeflow/fairing/blob/e0d44e870b467bbd773f836a8b1b648b79019dd1/kubeflow/fairing/kubernetes/manager.py#L220) to check for the existence of a secret instead of read_namespaced_secret.

The default pipeline-runner SA used by Kubeflow Pipelines does not have the list permission for [secrets](https://github.com/kubeflow/manifests/blob/9e6ef8681003c195560fb7b3b75211830c19b7a0/pipeline/pipelines-runner/base/cluster-role.yaml#L9), so fairing fails to check that the secret exists and fails to attach it. kubeflow/pipelines#3742

Correcting the secret_exists would fix the problem.

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