Stub in new SSL implemenation for Apple OS

[singpolyma]

Use native SSL on Apple OS instead of mbedtls. Especially needed on iOS where the root CA store cannot be read.

Fixes #570

[Aidan63]

I think using OS SSL libraries where possible is a good idea (I've got a pending merge in #1135 which does it for Windows) and have looked at this mac library before, I've got a few comments though.

• Secure Transport is deprecated and it is now recommended to use the Network Framework instead, this is important as Secure Transport does not support TLS 1.3. Hxcpp doesn't at the moment either but the pending mbedtls update and the Windows SChannel merges would add TLS 1.3 support, so this would be a step backwards for mac users.
• Do Apply have cryptographic libraries which can be used to implement all the hashing and digest functions? Would be nice to use those if possible as it would allow mac builds to not need mbedtls at all.
• Could we have a flag to switch back to mbedtls for mac? In WinCrypt and Friends SSL Implementation #1135 I have it set to use SChannel by default but the HXCPP_USE_MBEDTLS flag allows you to go back to mbedtls.

[barisyild]

I think best solution is using native http api of apple os.

The sample code is a nme wrapper, so it requires a few changes to compile.
Below is an example of a working haxe bridge, @:objc is required.

#if ios
private function iOSResponse(data:NSData, response:NSURLResponse, error:NSError)
{
	var statusCode = -1;
	if (response != null) {
		var httpResponse:NSHTTPURLResponse = untyped __cpp__("(NSHTTPURLResponse *) {0}", response);
		statusCode = httpResponse.statusCode;
	}

	var shouldRetry = (error != null) || (statusCode >= 500);

	if (shouldRetry && retryCount < maxRetries)
	{
		retryCount++;
		Sys.println('${urlRequest.url} request failed, retrying... Attempt ${retryCount} status code ${statusCode}');
		runiOS();
		return;
	}

	if (error != null || statusCode >= 400)
	{
		onError(error != null ? error.localizedDescription.toString() : '${statusCode}');
		return;
	}

	onStatus(statusCode);
	retryCount = 0;

	var bytes = data.toBytes();
	if (urlLoader.dataFormat== URLLoaderDataFormat.BINARY)
	{
		byteData = ByteArray.fromBytes(bytes);
	}
	else
	{
		stringData = bytes.getString(0, bytes.length, UTF8);
	}

	state = URLLoader.urlComplete;
}

public function runiOS() {
	var url:NSURL = NSURL.URLWithString(this.urlRequest.url);
	var request:NSMutableURLRequest = NSMutableURLRequest.requestWithURL(url);

	for(header in headers.keys())
	{
		var value:String = Std.string(headers.get(header));
		request.setValueForHTTPHeaderField(value, header);
	}

	var session:NSURLSession = NSURLSession.sharedSession();
	var task:NSURLSessionDataTask = session.dataTaskWithRequestCompletionHandler(request, untyped __cpp__('^(NSData *data, NSURLResponse *response, NSError *error) {
		dispatch_async(dispatch_get_main_queue(), ^{
			{0}(data, response, error);
		});
	}', iOSResponse));

	task.resume();
}
#end