`lib/gitlab/database/partitioning/` should work with many databases

Currently our partitioning is a hand written generation of different SQL queries executed on top of ActiveRecord::Base.connection. We should adapt that to properly support many databases, and make it to use a connection of an associated model for which partitions are to be created.

Ref.:

• https://gitlab.com/gitlab-org/gitlab/-/blob/a9c318847faad3b6db4e5d51440c009cff578dff/lib/gitlab/database/partitioning/monthly_strategy.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/a9c318847faad3b6db4e5d51440c009cff578dff/lib/gitlab/database/partitioning/partition_manager.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/a9c318847faad3b6db4e5d51440c009cff578dff/lib/gitlab/database/partitioning/replace_table.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/a9c318847faad3b6db4e5d51440c009cff578dff/lib/gitlab/database/partitioning/time_partition.rb

1. The first issue is that partitioned tables fail to create on the ci: database (config/initializers/postgres_partitioning.rb). This is tricky because only the audit_event structure exists on the ci: database but the AuditEvent model don't ever have a connection to that ci: database.
   1. Note: Partitioning creation/sync seems to work locally for some reason. We should investigate why