Merge run interfaces

[general-CbIC]

Improvements based on review comments from #59

Poolex.run!/3 was removed in favor of Poolex.run/3. The new unified function returns {:ok, result} or {:error, :checkout_timeout} and not handles runtime errors anymore.

• Reason: We should not catch errors in the caller process. The caller process itself must choose how to handle exceptions and exit signals.