Add: use https:// for content-service connections (#10448)

This requires the use of WinHTTP (for Windows) or libcurl (for all
others except Emscripten). Emscripten does not support http(s)
calls currently.

On Linux it requires ca-certificates to be installed, so the HTTPS
certificate can be validated. It is really likely this is installed
on any modern machine, as most connections these days are HTTPS.

(On MacOS and Windows the certificate store is filled by default)

Reminder: in case the http(s):// connection cannot be established,
OpenTTD falls back to a custom TCP-based connection to fetch the
content from the content-service. Emscripten will always do this.

src/network/network_content.cpp

@@ -311,13 +311,6 @@ void ClientNetworkContentSocketHandler::DownloadSelectedContent(uint &files, uin
 {
 	bytes = 0;
 
-#ifdef __EMSCRIPTEN__
-	/* Emscripten is loaded via an HTTPS connection. As such, it is very
-	 * difficult to make HTTP connections. So always use the TCP method of
-	 * downloading content. */
-	fallback = true;
-#endif
-
 	ContentIDList content;
 	for (const ContentInfo *ci : this->infos) {
 		if (!ci->IsSelected() || ci->state == ContentInfo::ALREADY_HERE) continue;
@@ -361,7 +354,7 @@ void ClientNetworkContentSocketHandler::DownloadSelectedContentHTTP(const Conten
 
 	this->http_response_index = -1;
 
-	new NetworkHTTPContentConnecter(NetworkContentMirrorConnectionString(), this, NETWORK_CONTENT_MIRROR_URL, content_request);
+	NetworkHTTPSocketHandler::Connect(NetworkContentMirrorUriString(), this, content_request);
 	/* NetworkHTTPContentConnecter takes over freeing of content_request! */
 }