feat: add product variant management functionality

- Implemented CreateProductVariantDto and UpdateProductVariantDto for product variant data transfer.
- Developed ProductVariantsController to handle CRUD operations for product variants.
- Created ProductVariantsService to interact with the database using Prisma.
- Added ProductVariantsModule to encapsulate the product variant feature.
- Included response mapping for consistent API responses.

src/customers/customers.service.ts

@@ -1,27 +1,33 @@
 import { Injectable } from '@nestjs/common'
+import { ResponseMapper } from '../common/response/response-mapper'
 import { PrismaService } from '../prisma/prisma.service'
 
 @Injectable()
 export class CustomersService {
   constructor(private prisma: PrismaService) {}
-
-  create(data: any) {
-    return this.prisma.customer.create({ data })
+  async create(data: any) {
+    const item = await this.prisma.customer.create({ data })
+    return ResponseMapper.create(item)
   }
 
-  findAll() {
-    return this.prisma.customer.findMany()
+  async findAll() {
+    const items = await this.prisma.customer.findMany()
+    return ResponseMapper.list(items)
   }
 
-  findOne(id: number) {
-    return this.prisma.customer.findUnique({ where: { id } })
+  async findOne(id: number) {
+    const item = await this.prisma.customer.findUnique({ where: { id } })
+    if (!item) return null
+    return ResponseMapper.single(item)
   }
 
-  update(id: number, data: any) {
-    return this.prisma.customer.update({ where: { id }, data })
+  async update(id: number, data: any) {
+    const item = await this.prisma.customer.update({ where: { id }, data })
+    return ResponseMapper.update(item)
   }
 
-  remove(id: number) {
-    return this.prisma.customer.delete({ where: { id } })
+  async remove(id: number) {
+    const item = await this.prisma.customer.delete({ where: { id } })
+    return ResponseMapper.single(item)
   }
 }