Add the gitlab_mr_diff schema and mr_diff connection

What does this MR do and why?

Adds the gitlab_mr_diff schema, the mr_diff database connection and MrDiffApplicationRecord. This is the first step in moving the merge request diff tables to their own database.

It changes nothing at runtime:

• The mr_diff connection uses fallback_database: main, and MrDiffApplicationRecord only calls connects_to when mr_diff is configured. Without it, the model uses main's connection pool.
• No tables use gitlab_mr_diff yet. The tables move in a later merge request.

Not included yet:

• The config/database.yml template for mr_diff and the CI variable that turns it on in CI. https://gitlab.com/gitlab-org/gitlab/-/work_items/635395 tracks them.
• The hardcoded database lists (the gitlab:db:migration_testing tasks, gitlab:db:truncate_legacy_tables:mr_diff, TablesTruncate, schema_load_fix, spec/support/multiple_databases.rb and loose foreign key turbo mode with its ops flag). They only run once CI configures mr_diff, so they go in the same merge request as the template.
• The background migration and background operation workers, the Sidekiq duration limit, and the Prometheus health indicator case. They only matter once mr_diff has its own connection, and the workers' cron entries and the Sidekiq limit would add work on every installation before then.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/635350
• Step-by-step checklist for a new decomposed database

How to set up and validate locally

1. With no mr_diff entry in config/database.yml, check the fallback in a Rails console:

   Gitlab::Database.has_config?(:mr_diff)            # => false
   MrDiffApplicationRecord.connection_db_config.name # => "main"
   Gitlab::Database.gitlab_schemas_for_connection(ApplicationRecord.connection).include?(:gitlab_mr_diff) # => true

2. Run the specs that check connection and schema definitions:

   bin/rspec spec/lib/gitlab/database_spec.rb spec/lib/gitlab/database/gitlab_schema_spec.rb spec/lib/gitlab/database/database_connection_info_spec.rb

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.