feat: scheduled service start

[hpidcock]

This introduces scheduled starting of services using the existing timer format
defined by snapd. To achieve this, a new field, schedule, is added to the
service layer schema. Additionally, the pebble services command has an extra
column/field, scheduled, which necessitated adding a complementary field to
the /v1/services GET endpoint.

The internal mechanism for starting services on a schedule is implemented in
internals/overlord/servstate/schedule.go, which utilises changes/tasks for
state keeping. The timer wakeup is driven through deferring the task into the
future, when that time passes the schedule is reassessed either deferring again
or adding a service start task to the change to be handled immediately.

An example of a one-shot service that runs periodically:

services:
  cron:
    override: replace
    startup: disabled
    command: ping -c 3 1.1.1.1
    schedule: 00:00-24:00/1440
    on-success: ignore

To see when it is scheduled next we can use the pebble services command:

$ pebble services
Service  Startup   Scheduled            Current   Since
cron     disabled  today at 19:55 AEST  inactive  today at 19:54 AEST

If we need to see more information on past runs, we can use the pebble changes
command:

$ pebble changes
ID   Status  Spawn                Ready                Summary
1    Done    today at 19:53 AEST  today at 19:53 AEST  Scheduled start of service "cron"
2    Done    today at 19:53 AEST  today at 19:54 AEST  Scheduled start of service "cron"
3    Doing   today at 19:54 AEST  -                    Scheduled start of service "cron"

It is then possible to dive deeper into the change to look at the tasks with
the pebble tasks command:

$ pebble tasks 2
Status  Spawn                Ready                Summary
Done    today at 19:53 AEST  today at 19:54 AEST  Wait for scheduled start of service "cron"
Done    today at 19:54 AEST  today at 19:54 AEST  Start service "cron"

......................................................................
Wait for scheduled start of service "cron"

2026-08-06T19:54:00+10:00 INFO Started service "cron" on schedule; next scheduled start at 2026-08-06T19:55:00+10:00.

References

• OP092 spec - Doc/Markdown
• Snapd Timer String Format