Add support for commandline option to update cores and other stuff (Feature Request)

Feature Request

Currently I can update the cores and assets through the GUI by simply clicking the icon. I imagine it would be an easy thing to add a commandline option like --update cores assets and so on to kick the exact same update procedure without using the GUI. This can be used to automate the update process through scripts, in example whenever I update my system. Here is an image of the section I am talking about: https://i.imgur.com/D1A3Pks.png

Description

No way to update cores and assets without using the GUI.

Expected behavior

Commandline option like --update cores assets, so the updating these parts would not require opening the GUI.

Version/Commit

• RetroArch: 1.9.0/5e551dd92b

retroarch --version
RetroArch: Frontend for libretro -- v1.9.0 -- 5e551dd92b --
Compiler: GCC (10.2.0) 64-bit Built: Jan  5 2021

Environment information

• OS: Manjaro Linux x86_64

Start by locating the command-line option parsing and the GUI action that updates cores and assets, then trace how the GUI invokes the existing update procedure. Add a command-line path for options such as --update cores assets that uses the same procedure, and verify that updates can run without opening the GUI.