docs: Add Swift SDK README with comprehensive API documentation and implement authentication provider with JWT support

- Add README.md with installation instructions, architecture overview, and API examples for all SDK modules
- Add AuthProvider class with API key and JWT token support (generateJWTToken, validateJWTToken, getHeaders, clearTokenCache methods)
- Add HTTPClient base class with retry logic, error handling, and timeout management
- Add GraphQLAPI, PaymentAPI, RatingPlanAPI, SubscriberAPI, SystemAPI, and UsageAPI modules
- Add TelecomSDK main client integrating all API

sdk/swift/Sources/TelecomSDK/PaymentAPI.swift

@@ -0,0 +1,42 @@
+import Foundation
+
+/// API for payment management
+public class PaymentAPI {
+    private let client: HTTPClient
+    
+    public init(client: HTTPClient) {
+        self.client = client
+    }
+    
+    /// Create a payment transaction
+    public func createTransaction(_ request: CreatePaymentRequest) async throws -> PaymentTransaction {
+        return try await client.post(path: "/v1/payments/transactions", body: request)
+    }
+    
+    /// Get a payment transaction by ID
+    public func getTransaction(transactionID: String) async throws -> PaymentTransaction {
+        return try await client.get(path: "/v1/payments/transactions/\(transactionID)")
+    }
+    
+    /// List payment transactions
+    public func listTransactions(
+        subscriberID: Int64? = nil,
+        status: PaymentStatus? = nil,
+        page: Int = 1,
+        pageSize: Int = 50
+    ) async throws -> SubscriberList {
+        var params: [String: String] = [
+            "page": "\(page)",
+            "page_size": "\(pageSize)"
+        ]
+        
+        if let subscriberID = subscriberID {
+            params["subscriber_id"] = "\(subscriberID)"
+        }
+        if let status = status {
+            params["status"] = "\(status)"
+        }
+        
+        return try await client.get(path: "/v1/payments/transactions", params: params)
+    }
+}