feat(backend): use cert-manager for cache server cert

[surajkota]

Description of your changes:

• Manifests for using cert-manager for the certificate required by cache-server
  • Created a cert-manager directory under env instead of adding the manifests under aws since this issue was also found on Mercedes Benz distribution of Kubernetes. This means with the current implementation(which does not use cert-manager), caching will not work on any k8s distribution which issues certificates for CSRs with signerName kubernetes.io/kubelet-serving only to kubelets
  • Addressed comments from feat(backend): Use cert-manager for multi-user aws #7779

Testing:

• Manually tested both standalone installation and multi user with Kubeflow installed with a custom built cache-server image since there are changes to backend/src/cache/main.go

[zijianjoy]

/retest-required

[zijianjoy]

You need to run manifests/gcp_marketplace/test/snapshots.sh to update snapshot

[zijianjoy]

Created #7852 to fix the mkp-snapshot failure

[google-oss-prow]

@surajkota: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipelines-component-yaml	b91a52d	link	true	/test kubeflow-pipelines-component-yaml
kubeflow-pipelines-components-google-cloud-python38	b91a52d	link	true	/test kubeflow-pipelines-components-google-cloud-python38

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[surajkota]

/test kubeflow-pipeline-upgrade-test

[zijianjoy]

#7852 is merged

[zijianjoy]

/lgtm
/approve

Thank you @surajkota !

CC @chensun as we need to cherry pick this and another PR to 1.8 branch and release a 1.8.2 for this.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zijianjoy

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[surajkota]

Thanks @akartsky for investigating this. Thanks @zijianjoy for prioritizing the review.

@chensun We will need the following 2 PRs to be cherry-picked for 1.8.2:
#7538
#7843

We might need this as well for the tests: #7852