Expose setting to specify config_command in GitLab Workhorse config file

What does this MR do?

Expose setting to specify config_command in GitLab Workhorse config file. Depends on gitlab!140898 (merged)

Testing

1. Deploy GitLab using the following docker-compose.yml file

   docker-compose.yml

   version: '3.8'
   name: 'gitlab-test'
   
   x-default_settings: &default_settings
     image: "${GITLAB_IMAGE:-registry.gitlab.com/gitlab-org/omnibus-gitlab/gitlab-ee:8363-workhorse-external-config}"
     pull_policy: always
     privileged: true
     shm_size: '256m'
     restart: always
   
   services:
     redis:
       <<: *default_settings
       environment:
         GITLAB_OMNIBUS_CONFIG: |
           roles ['redis_master_role']
   
           # Redis configuration
           redis['bind'] = '10.0.0.2'
           redis['port'] = 6379
           redis['password'] = 'redis-password'
   
           # This is a pure Redis node. We don't need Rails stuff.
           gitlab_rails['enable'] = false
       networks:
         default:
           ipv4_address: 10.0.0.2
       healthcheck:
         test: ["CMD", "gitlab-redis-cli", "ping"]
         start_period: 60s
     gitlab:
       <<: *default_settings
       environment:
         GITLAB_OMNIBUS_CONFIG: |
           external_url "http://127.0.0.1"
   
           # Disable bundled Redis
           redis['enable'] = false
   
           # Disable PG
           postgresql['enable'] = false
   
           # Disable everything else that uses Redis
           gitlab_rails['enable'] = false
           gitlab_kas['enable'] = false
           prometheus_monitoring['enable'] = false
   
           # Enable GitLab Workhorse
           gitlab_workhorse['enable'] = true
           # GitLab Exporter uses the Redis information from `gitlab_rails`
           gitlab_rails['redis_host'] = '10.0.0.2'
           gitlab_rails['redis_port'] = '6379'
       ports:
         - "80:80"
         - "443:443"
         - "22:22"
       networks:
         default:
           ipv4_address: 10.0.0.3
       healthcheck:
         disable: true
       depends_on:
         redis:
           condition: service_healthy
   
   networks:
     default:
       ipam:
         config:
           - subnet: 10.0.0.0/24

2. Check the logs of gitlab-workhorse and see that it shows a warning about Redis not having an auth

3. In the gitlab container, create the file /opt/redis-for-workhorse.sh with the following content

   #!/usr/bin/env bash
   
   cat <<EOF
   {
     "redis": {
       "password": "redis-password"
     }
   }
   EOF

4. Make it executable

   $ chmod +x /opt/redis-for-workhorse.sh

5. Add the following to /etc/gitlab/gitlab.rb

   gitlab_workhorse['extra_config_command'] = '/opt/redis-for-workhorse.sh'

6. Run gitlab-ctl reconfigure and see that the error in gitlab-workhorse logs regarding Redis auth is gone.

Related issues

Closes #8363 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.