mgr: add mgr_subinterpreter_modules config

mgr: add mgr_subinterpreter_modules config

This commit adds a mgr_subinterpreter_modules config to cause specified
modules (or all if * is specified) to be loaded in individual
subinterpreters.

This changes the default behavior of ceph-mgr from running each module
in a distinct subinterpreter to running them all in the same main
interpreter.  We can reintroduce subinterpreter support over time by
adding modules to the list as we test them.

Fixes: https://tracker.ceph.com/issues/73857
Fixes: https://tracker.ceph.com/issues/73859
Signed-off-by: Samuel Just <sjust@redhat.com>