Enter for Raw endpoints

Agreed with @phadej to open an issue after discussion on IRC.

I have regular Get, Put, Post endpoint, but also need to make use of a raw endpoint and I would like to access my `Reader` environment from the raw handler the same way that I can do it for regular servant handlers, but that is currently not possible. E.g. in the simplified example below from the handler of type `Application` I would like to ask for any of the config information e.g. database connection.

```
type Api = Raw

api :: ServerT Api MyHandler
api = Tagged $ handler
where
handler :: Application
handler = do
conn <- asks cfgDatabaseConnection -- currently not possible
runDB conn query

server :: Config -> Server Api
server cfg = enter (runReaderTNat cfg :: MyHandler :~> Handler) api
```