Introduce a new action `ms_loaded`

[flixos90]

There are several areas in Multisite which are available to be used in sunrise.php, but will lack specific functionality when used there.

Therefore, as per the discussion in #36935 (this discussion includes an example), we would like to introduce a new action ms_loaded that is run after the Multisite bootstrapping process has been completed. This action can then be used to conditionally detect whether specific functionality should be made available yet or not.

This action being introduced is necessary for the current state of the above ticket to go in as well, but it will certainly help for other use-cases as well in the future.

[flixos90]

37235.diff​ introduces the action. Flagging this for commit, as this ticket has mostly been opened for reference (see #36935) to add the action.

[jeremyfelt]

In 37916:

Multisite: Fire the ms_loaded action after multisite's bootstrap has finished.

This allows code in sunrise.php and in core to know when multisite bootstrap is complete and when the $current_site and $current_blog globals are populated.

Fixes #37235.
Props flixos90.