[release-1.17] Add the ability to auth via certs without storing them in etcd secret

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #5200

/assign enxebre

Allows Service Principal with Certificate authentication to work with a path to the certificate.

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[k8s-ci-robot]

@k8s-infra-cherrypick-robot: 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
pull-cluster-api-provider-azure-e2e-v1beta1	a6f53dd	link	true	/test pull-cluster-api-provider-azure-e2e-v1beta1
pull-cluster-api-provider-azure-build-v1beta1	a6f53dd	link	true	/test pull-cluster-api-provider-azure-build-v1beta1
pull-cluster-api-provider-azure-apiversion-upgrade-v1beta1	a6f53dd	link	true	/test pull-cluster-api-provider-azure-apiversion-upgrade-v1beta1
pull-cluster-api-provider-azure-e2e-aks-v1beta1	a6f53dd	link	true	/test pull-cluster-api-provider-azure-e2e-aks-v1beta1
pull-cluster-api-provider-azure-ci-entrypoint-v1beta1	a6f53dd	link	true	/test pull-cluster-api-provider-azure-ci-entrypoint-v1beta1
pull-cluster-api-provider-azure-test-v1beta1	a6f53dd	link	true	/test pull-cluster-api-provider-azure-test-v1beta1

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[nojnhuh]

I'm somewhat hesitant to backport any change to the API, even if we're confident it's backward-compatible.
/hold