[fs/core] Load external content without NAND install (#2862)

Adds the capability to add DLC and Updates without installing them to NAND. This was tested on Windows only and needs Android integration.

Co-authored-by: crueter <crueter@eden-emu.dev>
Co-authored-by: wildcard <wildcard@eden-emu.dev>
Co-authored-by: nekle <nekle@protonmail.com>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/2862
Reviewed-by: DraVee <dravee@eden-emu.dev>
Reviewed-by: crueter <crueter@eden-emu.dev>
Co-authored-by: Maufeat <sahyno1996@gmail.com>
Co-committed-by: Maufeat <sahyno1996@gmail.com>

src/common/settings.h

@@ -715,6 +715,7 @@ struct Values {
                                         Category::DataStorage};
     Setting<std::string> gamecard_path{linkage, std::string(), "gamecard_path",
                                        Category::DataStorage};
+    std::vector<std::string> external_content_dirs;
 
     // Debugging
     bool record_frame_times;