Goal

• As a RHEL Image Mode user, I want to use the familiar DNF interface to test packages on bootc systems without rebuilding and rebooting

Acceptance criteria

• User can use dnf4 --transient to perform transactions on bootc systems that reset on reboot
• It should be very clear to bootc users that changes are not going to persist across reboots like they would on regular mutable systems.
• It should be clear to non-bootc users that --transient doesn't work on their system.
• If /usr is already writable, DNF should proceed without complaint, even if the user doesn't explicitly specify --transient.
• Users should be able to make transient mode the default in dnf.conf, e.g. by setting persistence=transient.