MIAMI — In her just over 20 months as a federal judge, Aileen M. Cannon worked mostly in obscurity, becoming nominated and appointed to her position during the height of the coronavirus pandemic and at the end of a turbulent presidency. Then, last month, she was assigned the most prominent case of her short judicial career, involving the very person who put her on the bench: former President Donald J. Trump. On Monday, Judge Cannon granted Mr. Trump’s request to appoint an independent arbiter known as a special master to review materials seized last month from his private Florida club....