Full compatibility with the "ExistentialAny" upcoming feature (#8)

Discussed and requested in
apple/swift-openapi-generator#99 (comment)

Author: MahdiBM

Diff for: Package.swift

@@ -15,6 +15,13 @@
 import Foundation
 import PackageDescription
 
+// General Swift-settings for all targets.
+let swiftSettings: [SwiftSetting] = [
+    // https://github.com/apple/swift-evolution/blob/main/proposals/0335-existential-any.md
+    // Require `any` for existential types.
+    .enableUpcomingFeature("ExistentialAny")
+]
+
 let package = Package(
     name: "swift-openapi-async-http-client",
     platforms: [
@@ -39,13 +46,15 @@ let package = Package(
                 .product(name: "OpenAPIRuntime", package: "swift-openapi-runtime"),
                 .product(name: "AsyncHTTPClient", package: "async-http-client"),
                 .product(name: "NIOFoundationCompat", package: "swift-nio"),
-            ]
+            ],
+            swiftSettings: swiftSettings
         ),
         .testTarget(
             name: "OpenAPIAsyncHTTPClientTests",
             dependencies: [
                 "OpenAPIAsyncHTTPClient",
-            ]
+            ],
+            swiftSettings: swiftSettings
         ),
     ]
 )

Diff for: Sources/OpenAPIAsyncHTTPClient/AsyncHTTPClientTransport.swift

@@ -115,15 +115,15 @@ public struct AsyncHTTPClientTransport: ClientTransport {
     public var configuration: Configuration
 
     /// Underlying request sender for the transport.
-    internal let requestSender: HTTPRequestSending
+    internal let requestSender: any HTTPRequestSending
 
     /// Creates a new transport.
     /// - Parameters:
     ///   - configuration: A set of configuration values used by the transport.
     ///   - requestSender: The underlying request sender.
     internal init(
         configuration: Configuration,
-        requestSender: HTTPRequestSending
+        requestSender: any HTTPRequestSending
     ) {
         self.configuration = configuration
         self.requestSender = requestSender