sled agent: implement OmicronZoneImageSource::Artifact

[iliana]

Closes #7281.

[iliana]

This is a draft for now. I've been able to test this manually by crafting a new input to omicron_config_put to change the image source from install_dataset to artifact with the relevant hash; the zone gets torn down and rebuilt with the correct image path:

zonecfg:oxz_cockroachdb_2e4a6e55-3bc6-4ec1-ac53-9ad2c646047d> export
create -b
set zonepath=/pool/ext/d05bafeb-90c9-4a92-8254-6d93252a1c17/crypt/zone/oxz_cockroachdb_2e4a6e55-3bc6-4ec1-ac53-9ad2c646047d
set brand=omicron1
set autoboot=false
set limitpriv=default,dtrace_proc,dtrace_user
set ip-type=exclusive
add net
set physical="oxControlService29"
end
add dataset
set name="oxp_d05bafeb-90c9-4a92-8254-6d93252a1c17/crypt/cockroachdb"
end

I don't think I can test this end-to-end until there's also code which can store the zone image source in the database.

I also want to add code to prevent Sled Agent from accepting a zone config referring to artifacts it doesn't have per #7281 (comment).