Centralized Config Proxy Defaults dont support nested maps

[mkeeler]

In short making a PUT request to /v1/config with the following payload gets stored in raft but is unretrievable afterwards.

{
   "Kind": "proxy-defaults",
   "Name": "global",
   "Config": {
      "nested": {
         "foo": "bar"
      }
   }
}

mkeeler-corp:~ mkeeler$ curl -v localhost:8500/v1/config/proxy-defaults/global
*   Trying ::1...
* TCP_NODELAY set
* Connection failed
* connect to ::1 port 8500 failed: Connection refused
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8500 (#0)
> GET /v1/config/proxy-defaults/global HTTP/1.1
> Host: localhost:8500
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Vary: Accept-Encoding
< Date: Tue, 30 Apr 2019 15:03:33 GMT
< Content-Length: 53
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host localhost left intact
json: unsupported type: map[interface {}]interface {}

The problem seems to be the nested map internally is typed as a map[interface{}]interface{} which the JSON encoder refuses to handle.

To fix this I think we would need to not store these pass through configs as a regular map[string]interface{} but instead have some type which enforces the JSON compatibility of the pass through config when decoding/encoding and prevents these issues.

This is very loosely related to #4971 and could possibly be fixed in a common way.