Automatically create k8s secret when using `k8s_secret` variable for all pipelines types

Problem to solve

Currently GitLab provides a way to easily create k8s secrets for Auto DevOps pipelines by using the k8s_secret nomenclature with CI env variables, however, this functionality only works with Auto DevOps pipelines.

Intended users

developers, operators

Further details

GitLab security team is a possible candidate to dogfood this (poc are @melissar @agroleau @adietrich)

Proposal

Extend this functionality to all pipelines types, not just Auto DevOps.

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references