Add optional support for puma in the GitLab helm chart

Summary

The option to use Puma was recently added to Omnibus. The Helm charts don't currently have this option.

Problem statement

Without Puma support, the Helm Chart's functionality is trailing behind the options available in the Omnibus, and this includes other functionality that is only available with Puma like GitLab's Dependency Proxy.

Proposal

We should ship support for enabling puma. It should be optional, and not the default. It should be clearly documented that it is a currently unsupported feature.

To match omnibus support added in gitlab-org/omnibus-gitlab#3846 (closed) Related to gitlab-org&465 (closed)

As puma is already part of the gitlab-rails images, we probably want to branch off separate gitlab-puma-Xe images similar to the unicorn images.

This will also unblock https://gitlab.com/charts/gitlab/issues/1284