Adds plumbing to persist terraform plan data from template import provisioner builds.

After some deliberation, I went with storing what I have called thecached_plan in a separate table because these values are incredibly large, thetemplate_versions table already has a lot of columns, and a lot of queries usetemplate_version.* to get those many columns. Accidentally including thecached_plan column in those queries would be very undesirable, and this value mostly exists for future use on the backend. I don't think this is the sort of thing we would ever actually want to send to a client, except maybe a page for debugging the current state.

This also means that we can track it'supdated_at time separately quite easily, since we're planning on letting it be "refreshed".

Closes#17050