As reported in RHDHBUGS-169 and this internal Slack thread, adding new elements in the `extra*` fields requires repeating all the default ones that are defined in the rhdh-chart.

This boils down to how Helm works by design: it would override arrays - see https://github.com/helm/helm/issues/3767

There might be a way to explicitly override arrays, but we would need to have control over the upstream chart templates, which is not the case right now because we are using it as a subchart.

The goal of this spike task is to investigate a way to contribute something in the upstream chart that would support this use case of merging arrays.
If it works, we could contribute this to the upstream Chart.

NOTE: from the upstream chart perspective, it would be better if this change is backward compatible, so it doesn't break anyone.