Add dedicated limit counter for DAST Schedules

Follow-up from "DAST On-demand Scheduler - Implement Scheduling"

Add a counter for DAST on-demand scan as a part of the Limitable module.

We will have to add a limit for Maximum number of active schedules that can be created per project. Currently, multiple schedules can be created per project using different dast_site_profiles. After, this we should be able to limit the number of schedules that can be active per project.

References:

1. https://docs.gitlab.com/ee/user/gitlab_com/index.html#account-and-limit-settings
2. #29566 (closed)
3. https://docs.gitlab.com/ee/development/application_limits.html#testing-instance-wide-limits

The following discussion from !65327 (merged) should be addressed:

• @philipcunningham started a discussion: (+4 comments)

  question

  I notice that pipeline_schedule.rb includes Limitable and sets some daily limits. since we'll be triggering pipelines, do we need to be doing the same?