Draft: Spike JH using independent database

What does this MR do and why?

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2883

Use independent database for JH.

Spiked and implemented these functions:

• JH can use a separate postgresql database
• support such as `db:migrate:jh`, `db:rollback:jh` rake tasks now, `railt -T` will show all commands
• generated migration/schema files is placed into `jh/db/{migrate, schema_migrations}` folder
• JH models connect JH database successfully
• rails handle multiple models association with joins across multiple database
• JH tests run successfully

How to set up and validate locally

1. create jh/db/migrate and jh/db/schema_migrations folders

2. copy `jh/config/database.yml.example` contents into `config/database.yml`, modify it to fit your local development env

3. run `bundle exec rails db:create:jh` to create JH database

4. run `bundle exec rails db:migrate:jh` to perform 2 demo migrations, it should create `jh_cats` and `jh_balls` tables

5. run `bundle exec rails c` enter rails console, then can try these commands:

   # model connect JH database
   JH::Cat.count
   JH::Cat.create! name: 'Ruby'
   
   # model associations with join across multiple database
   a = Project.last
   a.cats.count
   a.cats.create! name: 'Ruby'
   a.cats
   
   b = a.group
   b.cats
   
   a.cats.first.balls.create! name: 'Macbook Air'
   a.cats.first.balls
   a.balls

MR acceptance checklist

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