feat: Adding endpoints to buyItem

spec.json

src/api/index.ts

src/api/models/ChallengeDTO.ts

src/api/models/ChallengeTemplateDTO.ts

src/api/models/CreateGoalDTO.ts

src/api/models/GoalDTO.ts

src/api/models/InventoryDTO.ts

src/api/models/ItemDTO.ts

src/api/models/MarkChallengeDTO.ts

src/api/models/ProgressDTO.ts

src/api/services/GoalService.ts

src/api/services/ItemService.ts

src/views/ShopView.vue