[chart] Support decomposed with internal PostgreSQL chart

Links:

• !2122 (merged)

How to spin up a VM to test GitLab charts with decomposed database setup

1. Create a GCP project with https://about.gitlab.com/handbook/infrastructure-standards/realms/sandbox/#how-to-get-started
2. Go to your GCP project in console.cloud.google.com. Note it may take 5-10 for your new GCP project to appear in the projects list
3. Go to the Compute Engine section, enable the API
4. Create a new VM instance. I chose Machine type: n2-standard-4. Considering choosing at least 20GB for the attached volume size
5. Download, and install gcloud locally. Select the SSH dropdown for the VM instance you want to connect to. Select the gcloud SSH option to get the command line to connect via SSH using the gcloud too. https://cloud.google.com/sdk/docs/install

The rest of the steps are coming soon

Proposal

Make https://gitlab.com/gitlab-org/charts/gitlab using internal PostgreSQL chart support decomposed database setup.

Note Charts + External DB already supports decomposed database setup