The response body of POST, GET /bridges should be the same as
the request body of the POST /bridges:
```
{"type": "mqtt",
"name": "my_mqtt_bridge"
}
```
We force the user to provide an Id of format `{type}:{name}` when GET,
DELETE, PUT a bridge:
`GET /bridges/{type}:{name}`
Since `gpb` is a GPL compile-time-only dependency, we should not
include it in the release.
Note that adding `gpb: :none` to the release applications list *will*
make Mix include its files *and* reference them in the startup +
release scripts...