[discuss] add custom extension fields to route, server, and upstream objects.

[YuanSheng Wang]

These objects of APISIX may be managed by a third-party platform, adding
custom extension fields to
allow the third-party management platform to store some personalized data
items.

the `extension` is the new field.

```
{
    "uri": "/index.html",
    "upstream": {
        ... ...
    },
    "extension" {
        ... ... # allow user to define
    }
}'
```

-- 

*MembPhis*
My GitHub: https://github.com/membphis
Apache APISIX: https://github.com/apache/incubator-apisix