Feature request: Allow to set session keyboard layout using localed

Feature summary

At the moment, the D-Bus interface "org.freedesktop.locale1" sets the keyboard layout for the next session. This is the intended and documented behavior:

Note that SetVConsoleKeyboard() instantly applies the new key mapping to the console,
while SetX11Keyboard() simply sets a default that may be used by later sessions.

In the Anaconda installer, we are looking for a way to set the keyboard layout for the current session across desktop environments:
https://bugzilla.redhat.com/show_bug.cgi?id=1955025#c26

At the moment of writing this:

• KWin allows to use the --locale1 command line option to make the SetX11Keyboard() D-Bus method change the current keyboard layout.
• A similar hook is in place for Sway
• @Conan_Kudo filed a request for Weston and Magpie.

It'd be nice to have a similar option available in Mutter.