Add custom_config for redis

What does this merge request do and why?

This MR adds an option to inject custom_config for redis without drastically changing GDK by adding new services for Redis in cluster/sentinel mode. See comment !2839 (comment 1192575777) for background.

For local verification

Add the following into gdk.yml and run gdk reconfigure.

redis:
  custom_config:
    test:
      cache:
        cluster:
          - host: '127.0.0.1'
            port: 7001
          - host: '127.0.0.1'
            port: 7101
          - host: '127.0.0.1'
            port: 7201

➜  gitlab-development-kit git:(sc1-custom-cluster-config) ✗ cat gitlab/config/redis.cache.yml
development: "unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=2"
test:
  cluster:
  - host: 127.0.0.1
    port: 7001
  - host: 127.0.0.1
    port: 7101
  - host: 127.0.0.1
    port: 7201

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.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.