Add ability to configure main_clusterwide database

What does this merge request do and why?

Add ability to configure main_clusterwide database. This is part of Cells effort. In previous GitLab MRs we added support for a different main_clusterwide database connection. The idea for main_clusterwide is to allow many Cells to share tables such as application_settings.

This only supports single cell. We will extend GDK config to support connection to second cell at later stage (you can see gitlab#388278 (comment 1381141501) for a rough idea)

Related: gitlab#388278 (closed)

How to set up and validate locally

1. Test config/database.yml by default (cells.enabled=false) with :

   make gitlab/config/database.yml

2. Test config/database.yml with cells.enabled=true) with :

   gdk config set cells.enabled true
   make gitlab/config/database.yml

With cells.enabled=true, you will see the main_clusterwide connection in config/database.yml:

(arm64) tkgl2:gdk-ee tkuah$ make gitlab/config/database.yml

ℹ️  'gitlab/config/database.yml' has incoming changes:
-------------------------------------------------------------------------------------------------------------
@@ -19,6 +19,14 @@ development:
     prepared_statements: false
     variables:
       statement_timeout: 120s
+  main_clusterwide: &main_clusterwide
+    adapter: postgresql
+    encoding: unicode
+    database: gitlabhq_development
+    database_tasks: false
+    host: /Users/tkuah/code/gdk-ee/postgresql
+    port: 5432
+    pool: 5
 
 test: &test
   main:
@@ -41,3 +49,6 @@ test: &test
     prepared_statements: false
     variables:
       statement_timeout: 120s
+  main_clusterwide:
+    <<: *main_clusterwide
+    database: gitlabhq_test

-------------------------------------------------------------------------------------------------------------
⚠️  WARNING: 'gitlab/config/database.yml' has been overwritten. To recover the previous version, run:

cp -f '/Users/tkuah/code/gdk-ee/.backups/gitlab__config__database.yml.20230508143904' \
'/Users/tkuah/code/gdk-ee/gitlab/config/database.yml'

If you want to protect this file from being overwritten, see:
https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/configuration.md#overwriting-configuration-files
-------------------------------------------------------------------------------------------------------------

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• [-] Documentation added/updated, if needed. => Next MR
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.