Rinse the turkey breast under cold water and pat it dry with paper towels. Place it on a cutting board.
In a small bowl, mix together the garlic powder, onion powder, dried thyme, dried rosemary, paprika, salt, and black pepper.
Rub the olive oil all over the turkey breast, then sprinkle the herb mixture evenly over the meat, making sure it is well-coated.
Place the turkey breast in the slow cooker. Pour the chicken broth around the turkey (not directly on top) to keep the spices intact.
If using, place the butter on top of the turkey breast to enhance the flavor and moisture.
Cover the slow cooker with the lid and set it to low heat. Cook for about 6 hours or until the internal temperature of the turkey reaches 165°F (75°C).
Once cooked, remove the turkey breast from the slow cooker and let it rest for 10-15 minutes before slicing.
For gravy, if desired, mix cornstarch with water in a small bowl and whisk until smooth. Stir this mixture into the remaining broth in the slow cooker, and cook on high for 10-15 minutes until thickened.
Slice the turkey breast and serve with the thickened gravy.