Move database partitioning code from initializers to module for reusing

What does this MR do and why?

Move database partitioning code from initializers to module for reusing.

This is trying to resolve the issue in gitlab-jh/gitlab#356 (comment 728224686)

Where we have an additional initialize (jh/config/initializers/postgres_partitioning.rb) will register another partitioned model, however, Partitioning.sync_partitions isn't called again with that in mind.

By extracting this code into a method, we can reuse it there. The merge request is in gitlab-jh/gitlab!172 (merged)

How to set up and validate locally

• git checkout v14.4.0 or git checkout v14.4.0-ee
• Note this is destructive to local database: bundle exec rake db:drop db:create db:structure:load db:migrate
• git checkout main-jh (the branch can be fetched from https://gitlab.com/gitlab-jh/gitlab/tree/main-jh)
• bundle config set --local gemfile jh/Gemfile
• bundle install
• bundle exec rake db:migrate
• Notice that db/structure.sql is changed.
• Running this will restore that: rake db:structure:dump because it'll run Gitlab::Database::Partitioning.sync_partitions before dumping the schema.

Here's the JiHu merge request to show that it's fixing the CI job db:check-schema over there: gitlab-jh/gitlab!172 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.