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/Invoice.h

@@ -16,7 +16,7 @@ private:
     ServiceBooking* m_booking;
     util::Timestamp m_invoiceDate;
     double m_laborCost;
-    util::Map<int, InventoryItem*> m_parts;
+    util::Map<std::string, InventoryItem*> m_parts;
     double m_partsCost;
     double m_discountPercentage;
     double m_totalAmount;
@@ -30,8 +30,8 @@ public:
         const std::string& bookingId,
         ServiceBooking* booking,
         const util::Timestamp& invoiceDate, 
-        double laborCost, const util::Map<int,
-        InventoryItem*>& parts,
+        double laborCost, 
+        const util::Map<std::string,InventoryItem*>& parts,
         double partsCost,
         double discountPercentage, 
         double totalAmount, 
@@ -44,7 +44,7 @@ public:
     ServiceBooking* getBooking() const;
     const util::Timestamp& getInvoiceDate() const;
     double getLaborCost() const;
-    const util::Map<int, InventoryItem*>& getParts() const;
+    const util::Map<std::string, InventoryItem*>& getParts() const;
     double getPartsCost() const;
     double getDiscountPercentage() const;
     double getTotalAmount() const;
@@ -56,7 +56,7 @@ public:
     void setBooking(ServiceBooking* booking);
     void setInvoiceDate(const util::Timestamp& invoiceDate);
     void setLaborCost(double laborCost);
-    void setParts(const util::Map<int, InventoryItem*>& parts);
+    void setParts(const util::Map<std::string, InventoryItem*>& parts);
     void setPartsCost(double partsCost);
     void setDiscountPercentage(double discountPercentage);
     void setTotalAmount(double totalAmount);