Two-time Oscar nominee Keira Knightley is known for being in “period pieces” such as “Pride and Prejudice,” so her playing the lead in the new film “Official Secrets,” scheduled to be released in the U.S. on Friday, may seem odd at first. That is until one considers that the time span being depicted — the early 2003 run-up to the invasion of Iraq — is one of the most dramatic and consequential periods of modern human history.