feat(crons): Add mark_unknown clock task

[evanpurkhiser]

This task will be triggered when we detect an anomaly in check-in
volume during a clock tick. When this happens we are unable to know that
all check-ins before that tick have been received and will need to mark
all in-progress monitors as resulting in a 'unknown' state.

Part of getsentry/sentry#79328

[github-actions]

versions in use:

The following repositories use one of the schemas you are editing. It is recommended to roll out schema changes in small PRs, meaning that if those used versions lag behind the latest, it is probably best to update those services before rolling out your change.

• getsentry/sentry: pip:sentry-kafka-schemas==0.1.111 (upgrade)

latest version: 0.1.113

There were changes to the JSON schema file, but we couldn't categorize any of
them. Therefore we don't know whether this change is safe to make.

This might be a gap in linting. Want to take a look at
https://github.com/getsentry/json-schema-diff/ and figure it out?

[wedamija]

I thought that when we detected an outage, we'd just set a variable somewhere so that when we go to mark a miss, we mark an unknown instead?

[evanpurkhiser]

Yeah we will. This is for marking all in progress as unknown status.

We need to do this when we detect an anomaly during a clock tick since it means there may be OK check-ins that were dropped and we would mark those check-ins as timed out once they reach their timeout_at later (after the volume has returned to normal).