Refactor customer domain structure and implement core entities, services, and handlers

- Introduced a flat structure for the customer domain, consolidating entities, aggregates, repositories, services, forms, and handlers into a single package.
- Added core entities: Customer and CustomerAggregate.
- Implemented request/response forms for customer authentication (Register, Login, Refresh Token, Change Password).
- Created CustomerService to handle business logic and data access through the Repository interface.
- Established CustomerHandler for HTTP request handling related to customer authentication.
- Removed previous domain structure artifacts to streamline the codebase.

internal/customer/form.go

@@ -0,0 +1,24 @@
+package customer
+
+// Customer Authentication DTOs
+type RegisterForm struct {
+	FirstName string `json:"first_name" valid:"required,alpha,length(2|50)"`
+	LastName  string `json:"last_name" valid:"required,alpha,length(2|50)"`
+	Email     string `json:"email" valid:"required,email"`
+	Mobile    string `json:"mobile,omitempty" valid:"optional,length(10|15)"`
+	Password  string `json:"password" valid:"required,length(8|128)"`
+}
+
+type LoginForm struct {
+	Email    string `json:"email" valid:"required,email"`
+	Password string `json:"password" valid:"required"`
+}
+
+type RefreshTokenForm struct {
+	RefreshToken string `json:"refresh_token" valid:"required"`
+}
+
+type ChangePasswordForm struct {
+	OldPassword string `json:"old_password" valid:"required"`
+	NewPassword string `json:"new_password" valid:"required,length(8|128)"`
+}