externals: httplib: replace custom httplib header with upstream as submodule.

This also includes a minor change to web_service.cpp - to fix compatibility with upstream changes.

.gitmodules

@@ -43,3 +43,6 @@
 [submodule "SDL"]
 	path = externals/SDL
 	url = https://github.com/libsdl-org/SDL.git
+[submodule "externals/cpp-httplib"]
+	path = externals/cpp-httplib
+	url = https://github.com/yhirose/cpp-httplib.git