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/GraphQLAPI.swift

@@ -0,0 +1,16 @@
+import Foundation
+
+/// API for GraphQL queries
+public class GraphQLAPI {
+    private let client: HTTPClient
+    
+    public init(client: HTTPClient) {
+        self.client = client
+    }
+    
+    /// Execute a GraphQL query
+    public func execute(query: String, variables: [String: Any]? = nil) async throws -> [String: Any] {
+        let request = GraphQLRequest(query: query, variables: variables)
+        return try await client.post(path: "/graphql", body: request)
+    }
+}