Allow instance owners to schedule planned downtime

[tn5421]

As it stands, this project is very good for detecting unplanned service outages, but there is currently not a way to distinguish between planned and unplanned outages.

[bahmanm]

Thanks 🙏 This is quite a valid requirement. I'll definitely get back to this once v1.0.0 is out.

[bahmanm]

Two pieces of feedback from the community so far:

1. There should be a way to declare the downtime w/o requiring a Google account.
   This can be possibly achieved by asking the interested admins to expose their schedule at a predefined URL which can be scraped by lemmy-meter. For example: http://INSTANCE/.well-known/host-metadata.json

2. Some sites have recurring planned downtime.
   This can be achieved by using the cron syntax to declare such periods.
   💡 In the case of a recurring interval competing w/ a non-recurring one, the latter wins.