feat: Add Vault as a Gitlab Managed Application

This implements gitlab-org/gitlab#9982 (closed) for installing and managing Hashicorp Vault as a Gitlab managed application for CI installed/managed Apps only.

A few points of note

• It relies on installing/using the helm-git plugin as the official hashicorp Vault chart is not in a helm repository
• We use the official Vault helm chart from hashicorp as it's being actively developed by hashicorp themselves
• After installation, you will still need to go into a pod and run vault operator init and vault operator unseal <unseal key> As per the standard Vault installation process. We can't easily automate this as there is a lot of specific information and configuration related to how to setup Vault specifically for your environment. E.g. Personally storing and distributing your unseal keys
• It's very likely that for production usage a user will need to configure the Vault chart with some specifics, e.g. setting the backing store to a cloud storage bucket, encryption, auto-unseal on startup