Add an option to disable mounting of token secrets

Add an option to disable mounting of token secrets

When secrets are managed externally and provided as environment variables we don't need to force users to create k8s secrets.

And if files exist, we actually override the environment variables.

We can't however just don't create mounted secrets, because it would break existing installations.

This MR adds a special option externallyManagedCiTokensEnvVariables which disables mounting secrets.

There were multiple community contributions trying to address this:

• Support for external secrets added via values.y... (!426 - merged)
• Support setting secrets via environment variables (!436 - closed)
• Adds new preRegistrationScript to run commands ... (!413)
• chore(gitlab-runner): add support to avoid moun... (!399 - closed)

What's the best way to test this MR?

What are the relevant issue numbers?