Add end point to get csr from tenant

[cniackz]

Fixes: https://github.com/miniohq/engineering/issues/736

End point to get csr from tenant:

$ k describe csr storage-tiny-tenant-tiny-csr
Name:               storage-tiny-tenant-tiny-csr
Labels:             v1.min.io/tenant=storage-tiny
Annotations:        <none>
CreationTimestamp:  Fri, 22 Apr 2022 15:57:25 -0400
Requesting User:    system:serviceaccount:minio-operator:minio-operator
Signer:             kubernetes.io/legacy-unknown
Status:             Approved,Issued
Subject:
  Common Name:    system:node:*.storage-tiny-hl.tenant-tiny.svc.cluster.local
  Serial Number:  
  Organization:   system:nodes
Subject Alternative Names:
         DNS Names:  storage-tiny-pool-0-0.storage-tiny-hl.tenant-tiny.svc.cluster.local
                     minio.tenant-tiny.svc.cluster.local
                     minio.tenant-tiny
                     minio.tenant-tiny.svc
                     *.
                     *.tenant-tiny.svc.cluster.local
Events:  <none>

$ curl 'http://localhost:9090/api/v1/namespaces/tenant-lite/tenants/storage-lite/csr' \
  -H 'Accept: */*' \
  -H 'Accept-Language: en-CA,en-GB;q=0.9,en-US;q=0.8,en;q=0.7' \
  -H 'Connection: keep-alive' \
  -H 'Cookie: token=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' \
  -H 'Referer: http://localhost:9090/namespaces/tenant-lite/tenants/storage-lite/summary' \
  -H 'Sec-Fetch-Dest: empty' \
  -H 'Sec-Fetch-Mode: cors' \
  -H 'Sec-Fetch-Site: same-origin' \
  -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.127 Safari/537.36' \
  -H 'sec-ch-ua: " Not A;Brand";v="99", "Chromium";v="100", "Google Chrome";v="100"' \
  -H 'sec-ch-ua-mobile: ?0' \
  -H 'sec-ch-ua-platform: "macOS"' \
  --compressed | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   135  100   135    0     0  17453      0 --:--:-- --:--:-- --:--:-- 33750
{
  "annotations": [],
  "name": "storage-lite-tenant-lite-csr",
  "resource_version": "962",
  "status": " Automatically approved by MinIO Operator"
}

.......................TestMain(): started
sleeping
start server
=== RUN   TestGetCSR
--- PASS: TestGetCSR (0.01s)
PASS
.......................TestMain(): completed

Because the changes requested were performed already