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 in this PR since @akartsky is on vacation

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

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[google-oss-prow]

@surajkota: The following test 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-pipeline-upgrade-test	b91a52d	link	true	/test kubeflow-pipeline-upgrade-test

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[surajkota]

/check cla

[surajkota]

/check-cla
/retest

[surajkota]

some issue with cla bot, will try opening a new PR