Implement the registry db queries to get and categorize the background_migrations statuses

Create the necessary db queries to determine and categorize the state and the counts of each background migration. In a particular we need a function that is able to provide the following information :

• Count of active background migrations
• Count of paused background migrations
• Count of failed background migrations
• Count of completed background migrations

this information will be needed to support Expose registry `/gitlab/v1/admin/db/migration/... (#1457)