WASHINGTON — Steven Mnuchin has raised $2.5 billion at his new private equity fund, according to people familiar with the matter, attracting investments from sovereign wealth funds in the Middle East, including Saudi Arabia, where he traveled extensively as Treasury secretary. The fund was started this year by Mr. Mnuchin, who served as former President Donald J. Trump’s Treasury secretary for his entire term, and is focused on technology and financial services investments. This summer, Mr. Mnuchin opened an office in Tel Aviv, and he has raised money from Saudi Arabia’s Public Investment Fund.