CHARLESTON, S.C., Feb 15 (Reuters) - Former UN ambassador Nikki Haley is expected to focus on the threats China and Russia pose to the United States and the need for fresh blood atop the Republican ticket in the first stop of her campaign for the 2024 presidential nomination on Wednesday. Haley is just the second declared candidate seeking the Republican nod to challenge Democratic President Joe Biden in 2024, allowing her to stand out in a so-far uncrowded field but also exposing her to the anger of Donald Trump, who is seeking a return to the White House. The 51-year-old...