GH-3392 dirty initial MSA support that shares logic with Mojang flows

Both act as the first step of AuthContext.

buildconfig/BuildConfig.cpp.in

@@ -35,6 +35,7 @@ Config::Config()
     PASTE_EE_KEY = "@MultiMC_PASTE_EE_API_KEY@";
     IMGUR_CLIENT_ID = "@MultiMC_IMGUR_CLIENT_ID@";
     META_URL = "@MultiMC_META_URL@";
+    MSA_CLIENT_ID = "@MultiMC_MSA_CLIENT_ID@";
 
     BUG_TRACKER_URL = "@MultiMC_BUG_TRACKER_URL@";
     DISCORD_URL = "@MultiMC_DISCORD_URL@";