Adopt a custom shared client

Sources/OpenAPIAsyncHTTPClient/AsyncHTTPClientTransport.swift

@@ -68,14 +68,22 @@ public struct AsyncHTTPClientTransport: ClientTransport {
         /// The HTTP client used for performing HTTP calls.
         public var client: HTTPClient
 
+        /// The default shared HTTP client.
+        ///
+        /// This is a workaround for the lack of a shared client
+        /// in AsyncHTTPClient. Do not use this value directly, outside of
+        /// the `Configuration.init(client:timeout:)` initializer, as it will
+        /// likely be removed in the future.
+        public static let sharedClient: HTTPClient = .init()
+
         /// The default request timeout.
         public var timeout: TimeAmount
 
         /// Creates a new configuration with the specified client and timeout.
         /// - Parameters:
         ///   - client: The underlying client used to perform HTTP operations.
         ///   - timeout: The request timeout, defaults to 1 minute.
-        public init(client: HTTPClient = .init(), timeout: TimeAmount = .minutes(1)) {
+        public init(client: HTTPClient = Self.sharedClient, timeout: TimeAmount = .minutes(1)) {
             self.client = client
             self.timeout = timeout
         }

Tests/OpenAPIAsyncHTTPClientTests/Test_AsyncHTTPClientTransport.swift

@@ -82,16 +82,8 @@ class Test_AsyncHTTPClientTransport: XCTestCase {
     }
 
     func testSend() async throws {
-        let eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: 1)
-        let httpClient = HTTPClient(
-            eventLoopGroupProvider: .shared(eventLoopGroup),
-            configuration: .init()
-        )
-        defer {
-            try! httpClient.syncShutdown()
-        }
         let transport = AsyncHTTPClientTransport(
-            configuration: .init(client: httpClient),
+            configuration: .init(),
             requestSender: TestSender.test
         )
         let request: OpenAPIRuntime.Request = .init(