Add Sidekiq Cron timezone override configuration

What does this MR do and why?

Allows setting up a timezone override to sidekiq-cron, so each job scheduled for 3 AM runs in the correct timezone (which may be different then the one set for the rails application in environments like GitLab Dedicated).

This includes:

• Migration
• Models
• API

This does not include Admin UI frontend changes

References

Part of #582262

How to set up and validate locally

1. Export an administrator personal access token (adjust the host if needed):

   export GITLAB_TOKEN=<admin_token>
   export GITLAB_URL=http://localhost:3000

2. Read the current settings and confirm the new attribute defaults to null:

   curl -Ss --header "PRIVATE-TOKEN: $GITLAB_TOKEN" \
     --url "$GITLAB_URL/api/v4/application/settings" | jq '.sidekiq_timezone_override'
   # => null

3. Confirm an invalid IANA identifier is rejected with 400 Bad Request:

   curl -Ss --request PUT --header "PRIVATE-TOKEN: $GITLAB_TOKEN" \
     --url "$GITLAB_URL/api/v4/application/settings?sidekiq_timezone_override=Not/AZone"
   # => {"message":{"sidekiq_timezone_override":["must be a valid IANA timezone identifier"]}}

4. Set a valid timezone and confirm it is returned:

   curl -Ss --request PUT --header "PRIVATE-TOKEN: $GITLAB_TOKEN" \
     --url "$GITLAB_URL/api/v4/application/settings?sidekiq_timezone_override=America/Chicago" \
     | jq '.sidekiq_timezone_override'
   # => "America/Chicago"

5. In a terminal, confirm the cron jobs pick up the override (timezone appended to each cron string):

   gdk restart rails-background-jobs
   gdk tail rails-background-jobs # wait until it is fully boot up, then control C to exit
   bundle exec rails runner 'puts Gitlab::SidekiqConfig::CronJobs.config.jobs.values.first["cron"]'
   # => 3-59/10 * * * * America/Chicago

6. Clear the override by sending a blank value through the API and confirm it resets to ``:

   curl -Ss --request PUT --header "PRIVATE-TOKEN: $GITLAB_TOKEN" \
     --url "$GITLAB_URL/api/v4/application/settings?sidekiq_timezone_override=" \
     | jq '.sidekiq_timezone_override'
   # => ''

7. In a terminal, confirm no timezone token is appended once the override is blank:

   gdk restart rails-background-jobs
   gdk tail rails-background-jobs # wait until it is fully boot up, then control C to exit
   bundle exec rails runner 'puts Gitlab::SidekiqConfig::CronJobs.config.jobs.values.first["cron"]'
   # => 3-59/10 * * * * America/Chicago

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