MgsUpdateDriver support for RoT update

[davepacheco]

Similar to #7988. This will involve the same steps mentioned there, plus a couple of others:

• updating the PendingMgsUpdate type to have a variant for the RoT [mgs] PendingMgsUpdate type to have a variant for the RoT #8054
• implementing a new SpComponentUpdateHelper for the RoT [reconfigurator] Prechecks and post-update actions for RoT updater #8157
• updating MgsUpdateDriver to create the new update helper when it sees a config for updating the RoT [mgs] PendingMgsUpdate type to have a variant for the RoT #8054
• update reconfigurator-sp-updater to support specifying this, analogous to how it does sp today [reconfigurator] Add RoT options for reconfigurator-sp-updater #8193
• the new variant used in PendingMgsUpdate will need to describe expected conditions for the RoT. This will include the contents of the A slot, the contents of the B slot, which one is active, and maybe some of the other fields related to which slot is active? [mgs] PendingMgsUpdate type to have a variant for the RoT #8054
• [ ] either the ArtifactCache or the update helper will have to crack open the tarball and pull out either the A image or B image, depending on which slot is being updated. I was figuring we'd add a new helper method to SpComponentUpdateHelper called something like load_artifact(&self, update: &PendingMgsUpdate, artifacts: Arc<ArtifactaCache>) -> Result<Vec<u8>>. For the SP and RoT bootloader, this would just call artifact_contents like today. For the RoT, it would first do that, extract the right image from the tarball, and hand that back. I believe the repo unpacking process already separates these so we don't have to worry about these at this level.