Accept :ok as successful return for Repo.transact/2 fun

[nirev]

This PR adds :ok as an accepted result for the fun passed to Repo.transact/2.

It is sometimes useful to return just :ok when you don't care about the results of the function, just that the sequence of operations executed successfully.

Eg:

def do_something do
  Repo.transact(fn -> 
    {:ok, x} = get_x_with_lock()
    {:ok, _} = update_all_records(x)
    
     :ok
  end)
end

[warmwaffles]

In practice I've had to do {:ok, :ok} to achieve this or just return something. Typically though I've thought of it as forcing you to think about the side effect that is being caused and having to conciously return the meaningful mutation.

[nirev]

Closed this as I realized this would implicate in way more changes than it was worth it