[bug#60497] [PATCH] services: unattended-upgrade: Add 'expression' field.

[goodoldpaul]

Dear Guixers,

I'm sending a patch to add an expression field to the 
unattended-upgrade-configuration record. The main point of this field is 
to be passed to guix system reconfigure -e .

This change should not break any existing configurations but I may have 
missed something.

Thank you for your time and efforts,

giacomo