`az container exec` has no way to pass arguments

There is no way to specify arguments in `az container exec`. For example:

az container exec -g ... -n ... --exec-command "ls /"

will just complain about not finding the `ls /` executable. There should really be some way to pass arguments instead of using a shell for doing anything.

(As it is right now, the comment "The most common use case is to open an interactive bash shell" is really misleading, since running a shell is just about the only useful thing you can do when you cannot pass arguments.)

(This bug is a superset of #8687)