Show sds config name in /certs when Secret Discovery Service is used

Title: Show sds config name in /certs when Secret Discovery Service is used

Description: the /cert admin path shows the certificate name used in Envoy and the this is very useful for debugging as it refers to the certificate used in the listener/cluster tls config.

The following is the /certs output when SDS is enabled:

...
   "ca_cert": [
    {
     "path": "\u003cinline\u003e",
     "serial_number": "116db900f6bb4bd4a3fcd8fa57a1b141",
     "subject_alt_names": [],
     "days_until_expiration": "36196",
     "valid_from": "2018-04-25T21:11:33Z",
     "expiration_time": "1982-03-19T15:43:17Z"
    }
   ],
   "cert_chain": [
    {
     "path": "\u003cinline\u003e",
     "serial_number": "0287be20c0bc92ce9d87b5f5f396f2ef1d7670",
     "subject_alt_names": [
      {
       "uri": "my-service-1"
      }
     ],
     "days_until_expiration": "0",
     "valid_from": "2019-03-18T22:43:59Z",
     "expiration_time": "2019-03-19T22:48:59Z"
    }
   ]
  },
  {
   "ca_cert": [
    {
     "path": "\u003cinline\u003e",
     "serial_number": "116db900f6bb4bd4a3fcd8fa57a1b141",
     "subject_alt_names": [],
     "days_until_expiration": "36196",
     "valid_from": "2018-04-25T21:11:33Z",
     "expiration_time": "1982-03-19T15:43:17Z"
    }
   ],
   "cert_chain": [
    {
     "path": "\u003cinline\u003e",
     "serial_number": "0287be20c0bc92ce9d87b5f5f396f2ef1d7670",
     "subject_alt_names": [
      {
       "uri": "my-service-2"
      }
     ],
     "days_until_expiration": "0",
     "valid_from": "2019-03-18T22:43:59Z",
     "expiration_time": "2019-03-19T22:48:59Z"
    }
   ]
  },
...

When SDS is enabled, only <inline> is showed for all certs in the dump which makes the /certs page kind of useless as you cannot link it back to the sds config name in listener/cluster tls config.

Expected Behavior: The sds config name should be used in the /certs output. https://github.com/envoyproxy/envoy/blob/34532dd6d636e5b0f6cbc42e84c554a78eb34b1d/api/envoy/api/v2/auth/cert.proto#L367

@JimmyCYJ @lizan @htuch thoughts?