Add support for the 'current open changes' generated changelog file

[Garanas]

Description

Needs: https://github.com/FAForever/fa/issues/6239 and https://github.com/FAForever/fa/issues/6240

Add the feature to support the 'current open changes' changelog file that is generated by #6240. The file would always be at the top. The file may not always exist - for example on deploy/faf the file would not exist. If the file exists it will always be at the same place with the same name, you can verify that a file exists using the user-global DiskGetFileInfo.

Additional features, such as perhaps a special format can be done in future pull requests. The only focus here is to show the generated file in-game.