Add tracer-sidecar and "Sometimes forks" assertion

[Anviking]

newtype State = State
  { hasSeenAFork :: Bool -- whether or not any node has seen a fork
  }

initialState :: IO State
initialState = do
  writeSdkJsonl sometimesForksDeclaration
  return $ State False

processMessage :: LogMessage -> State -> IO State
processMessage LogMessage{datum} state = case (datum, state) of
  (SwitchedToAFork{}, s@(State False)) -> do
    writeSdkJsonl sometimesForksReached
    return $ s { hasSeenAFork = True }
  (_, s) -> return s

[Anviking]

Merging into the underlying PR.