refactor: observation handler to AppConfig

With this:

- Is no longer necessary to pass observer as an argument
  to every function that needs observations.
- We can invoke the observer on every function that uses AppConfig.
  However it'd be better to just call the observer in the upper modules
  (like on App.hs).

test/spec/SpecHelper.hs

@@ -141,6 +141,7 @@ baseCfg = let secret = Just $ encodeUtf8 "reallyreallyreallyreallyverysafe" in
   , configRoleIsoLvl                = mempty
   , configInternalSCSleep           = Nothing
   , configServerTimingEnabled       = True
+  , configObserver                  = const $ pure ()
   }
 
 testCfg :: AppConfig