[reconfigurator] rendezvous subtask for marking VMMs to be stopped for update

[karencfv]

Related: #11169

[karencfv]

@sunshowers I have a question for you. This is the first time I write a rendezvous subtask so I might just be missing some context.

In #11127 I added a stopped_for_update_disposition_generation column to the vmm table so that we can mark each VMM directly.

After reading https://rfd.shared.oxide.computer/rfd/0541#_proposal_reconciliation_rpw and taking a look at #11115 , I understand that I shouldn't have added the column to that table at all and instead I should have created a separate rendezvous table.

You suggested using a rendezvous subtask, but the thing is, I don't think we can follow that approach here. We do want the vmm table to have the additional row. Otherwise we would have to change a lot of the code. As part of #11169 we'll be updating the instance update saga, and adapting the reincarnation background task, both of which use the vmm table as the source of truth.

In addition, https://rfd.shared.oxide.computer/rfd/0541#_creating_rows_in_rendezvous_tables the resource exisiting in inventory. I don't think VMMs should be collected as part of an inventory collection, there are just too many.

I think I'd rather go with a background task, thoughts?