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.

COMPILING.md

@@ -10,14 +10,17 @@ OpenTTD makes use of the following external libraries:
 - (encouraged) libpng: making screenshots and loading heightmaps
 - (optional) liblzo2: (de)compressing of old (pre 0.3.0) savegames
 
-For Linux, the following additional libraries are used (for non-dedicated only):
+For Linux, the following additional libraries are used:
 
+- (encouraged) libcurl: content downloads
 - libSDL2: hardware access (video, sound, mouse)
 - libfreetype: loading generic fonts and rendering them
 - libfontconfig: searching for fonts, resolving font names to actual fonts
 - libicu: handling of right-to-left scripts (e.g. Arabic and Persian) and
    natural sorting of strings
 
+If you are building a dedicated-server only, you don't need the last four.
+
 OpenTTD does not require any of the libraries to be present, but without
 liblzma you cannot open most recent savegames and without zlib you cannot
 open most older savegames or use the content downloading system.