Add Discord Rich Presence Support (#3883)

* Initial Discord RPC support

Build with Discord Presence ON

Fix RPC detection

Fix Time elapsed on pause; will now continue to count.

* Fix CI builds with compile flag

Addressed reviews

Fix silly mistakes

Fix 'Not in-game' display

class instead of namespace

Fix

Revamped

remove redundant code

Using Pimpl pattern

* Implement Null class

* Fix config updation

* Addressed All Reviews

* externals/discord-rpc : Updated to latest commit

CMakeLists.txt

@@ -20,6 +20,8 @@ option(ENABLE_WEB_SERVICE "Enable web services (telemetry, etc.)" ON)
 
 option(ENABLE_CUBEB "Enables the cubeb audio backend" ON)
 
+option(USE_DISCORD_PRESENCE "Enables Discord Rich Presence" OFF)
+
 if(NOT EXISTS ${CMAKE_SOURCE_DIR}/.git/hooks/pre-commit)
     message(STATUS "Copying pre-commit hook")
     file(COPY hooks/pre-commit