Make Task.logger accessible to delegate implementations outside of Package

Motivation:

This change was proposed in issue
[#389](#389).
Users writing their own `HttpClientResponseDelegate` implementation
might want to emit log messages to the `task`'s `logger`.

Modifications:

Changed the access level of `Task`'s `logger` property from `internal`
to `public`.

Result:

Users can access the `logger` property of a `Task` outside of the
`async-http-client`.

Author: felixschlegel

Sources/AsyncHTTPClient/HTTPHandler.swift

@@ -565,9 +565,9 @@ extension HTTPClient {
     public final class Task<Response> {
         /// The `EventLoop` the delegate will be executed on.
         public let eventLoop: EventLoop
+        public let logger: Logger // We are okay to store the logger here because a Task is for only one request.
 
         let promise: EventLoopPromise<Response>
-        let logger: Logger // We are okay to store the logger here because a Task is for only one request.
 
         var isCancelled: Bool {
             self.lock.withLock { self._isCancelled }