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.
* Changes all public-facing codersdk types to use a plain int64 (milliseconds) instead of time.Duration.
* Makes autostart_schedule a *string as it may not be present.
* Adds a utils/ptr package with some useful methods.
