This PR adds fields to templates that constrain values for workspaces derived from that template.
- Autostop: Adds a field max_ttl on the template which limits the maximum value of ttl on all workspaces derived from that template. Defaulting to 168 hours, enforced on edits to workspace metadata. New workspaces will default to the templates's `max_ttl` if not specified.
- Autostart: Adds a field min_autostart_duration which limits the minimum duration between successive autostarts of a template, measured from a single reference time. Defaulting to 1 hour, enforced on edits to workspace metadata.
