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/RatingPlanAPI.swift

@@ -0,0 +1,20 @@
+import Foundation
+
+/// API for rating plan management
+public class RatingPlanAPI {
+    private let client: HTTPClient
+    
+    public init(client: HTTPClient) {
+        self.client = client
+    }
+    
+    /// List all available rating plans
+    public func list() async throws -> [RatingPlan] {
+        return try await client.get(path: "/v1/rating-plans")
+    }
+    
+    /// Get a rating plan by ID
+    public func get(planID: String) async throws -> RatingPlan {
+        return try await client.get(path: "/v1/rating-plans/\(planID)")
+    }
+}