Implement Generate Invoice

<UserStory> PAY001: Generate Invoice </UserStory>

<Changes>
    1. Added Utility.h to project configuration for supporting invoice generation utilities.
    2. Updated Invoice model to use string-based keys for parts mapping instead of integer keys.
    3. Implemented PaymentManagementService::generateInvoice to aggregate labour cost, parts cost, and apply discounts.
    4. Integrated invoice creation with Factory to instantiate Invoice objects and persist them into datastore.
    5. Enhanced Enums with PaymentMode::NOTSET to handle default invoice state.
</Changes>

<Test>

 Acceptance Criteria:
 1. Invoice auto-generates for each service booking once jobs are completed.
 2. Invoice shows a clear breakdown of charges including labour cost, parts cost, discount, and total amount.

 Precondition:
  1. Service booking exists with at least one service and required inventory items.
  2. Datastore is available for storing invoices.
  3. Payment mode and status enums are properly configured.

 Steps:
  1. Complete all jobs in a service booking.
    - Verify that PaymentManagementService::generateInvoice is triggered.
  2. Check datastore for newly created invoice.
    - Verify that invoice contains booking ID, labour cost, parts cost, discount, and total amount.
  3. Inspect invoice details.
    - Verify that breakdown of charges is accurate and discount is applied correctly.
  4. Confirm invoice status.
    - Verify that invoice is created with PaymentMode::NOTSET and PaymentStatus::PENDING.
</Test>

<Review>
Sreeja Reghukumar, please review
</Review>

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/models/JobCard.h

@@ -1,12 +1,12 @@
 #pragma once
 #include <string>
+#include "Enums.h"
 #include "Timestamp.h"
 
 class ServiceBooking;
 class Service;
 class User;
 
-enum class ServiceJobStatus : int;
 
 class JobCard
 {
@@ -20,7 +20,7 @@ private:
     std::string m_technicianId;
     User* m_technician;
     util::Timestamp m_assignedDate;
-    ServiceJobStatus m_status;
+    util::ServiceJobStatus m_status;
     util::Timestamp m_completionDate;
 
 public:
@@ -32,7 +32,7 @@ public:
         const std::string& technicianId, 
         User* technician, 
         const util::Timestamp& assignedDate, 
-        ServiceJobStatus status,
+        util::ServiceJobStatus status,
         const util::Timestamp& completionDate
     );
     const std::string& getId() const;
@@ -43,7 +43,7 @@ public:
     const std::string& getTechnicianId() const;
     User* getTechnician() const;
     const util::Timestamp& getAssignedDate() const;
-    ServiceJobStatus getStatus() const;
+    util::ServiceJobStatus getStatus() const;
     const util::Timestamp& getCompletionDate() const;
     void setId(const std::string& id);
     void setBookingId(const std::string& bookingId);
@@ -53,6 +53,6 @@ public:
     void setTechnicianId(const std::string& technicianId);
     void setTechnician(User* technician);
     void setAssignedDate(const util::Timestamp& assignedDate);
-    void setStatus(ServiceJobStatus status);
+    void setStatus(util::ServiceJobStatus status);
     void setCompletionDate(const util::Timestamp& completionDate);
 };