Combine the softened butter, garlic, lemon zest, rosemary, thyme, sage, paprika, salt, and black pepper in a small bowl and mash together with a fork to create a uniform herb butter. Pat the turkey breast completely dry using paper towels, leaving the skin intact. Gently loosen the skin from the meat without detaching it entirely, then apply half the herb butter underneath the skin, massaging it evenly across the surface of the meat. Spread the remaining butter across the outer surface of the skin. Pour the chicken broth into a 6-quart slow cooker and add the bay leaves, onion, halved garlic head, lemon quarters, thyme sprigs, and peppercorns. Position the seasoned turkey breast atop the aromatics, skin-side up. Cover and cook until the internal temperature reaches 165°F at the thickest point of the breast, approximately 4 to 5 hours on LOW or 3 to 4 hours on HIGH. Preheat the oven broiler to HIGH and position a rack in the middle of the oven. Move the cooked turkey breast to a rimmed baking sheet and broil for 7 to 9 minutes until the skin turns golden and crisp. Transfer the turkey to a cutting board and allow it to rest for 15 to 20 minutes to enable the juices to redistribute. Carve by slicing along each side of the breastbone, following the curve of the wishbone and cutting close to the ribs to remove the breast meat, then slice crosswise.