Lightly spray a 6-quart slow cooker with cooking spray. In a mixing bowl, whisk together egg white and buttermilk. In another bowl, add spices to flour. Place breadcrumbs on a large plate. Dip thighs, one at a time, in egg mixture and allow the excess to drip off. Then dip into flour and again into egg mixture. Coat with crumbs and press into chicken. Repeat for each thigh. Spray lightly each thigh, on both sides, with cooking spray and place side by side in slow cooker. Lightly sprinkle the top of each thigh with extra black pepper. Cover and cook on high 2 1/2 hours or until thighs have reached an internal temperature of 165°F. Remove lid and continue cooking for 10 minutes to allow chicken coating to get crunchy.