[DISCUSS] Add control api support for the discovery module

[聂永]

Hi,

I’m using the custom discovery module consuk_kv.lua online now.

And I often need the module to export some data snapshot in memory when running 
for debugging, below is the consuk_kv.lua export one api output.

# curl -s http://127.0.0.1:9090/v1/discovery/consul_kv/dump | jq
{
  "http://172.19.5.30:8500/v1/kv/upstreams/10000/webpages/": [
    {
      "host": "172.19.5.12",
      "port": 8000,
      "weight": 120
    },
    {
      "host": "172.19.5.13",
      "port": 8000,
      "weight": 120
    }
  ],
  "http://172.19.5.30:8500/v1/kv/upstreams/1614480/webpages/": [
    {
      "host": "172.19.5.12",
      "port": 8000,
      "weight": 120
    },
    {
      "host": "172.19.5.13",
      "port": 8000,
      "weight": 120
    }
  ],
  "http://172.19.5.31:8500/v1/kv/upstreams/1614480/webpages/": [
    {
      "host": "172.19.5.12",
      "port": 8000,
      "weight": 120
    },
    {
      "host": "172.19.5.13",
      "port": 8000,
      "weight": 120
    }
  ],
  "http://172.19.5.30:8500/v1/kv/upstreams/1614480/grpc_upstreams/": [
    {
      "host": "172.19.5.51",
      "port": 50051,
      "weight": 1
    }
  ]
}



What do you think?

If We need, I will submit a PR later :))