PWB: Support defining sensitive config values via k8s secrets

[pat-s]

Applies to

• launcherPem
• secureCookieKey
• userPassword
• database.conf

When doing so, the rstudio-secret is skipped and individual mounts containing the above-mentioned secret values are injected.

This PR is breaking as the items listed are now maps and existing deployments would need to migrate from launcherPem: <value> to launcherPem.value: <value>.

I've tested this in a deployment of mine and it works as expected so far.

secureCookieKey:
  existingSecret: secure-cookie-key
launcherPem:
  existingSecret: launcher-pem
config:
  database:
    conf:
      existingSecret: database-conf
userPassword:
  existingSecret: workbench-user-password

The advantage of this approach is that it gives power to the users and makes use of "simple" k8s secrets instead of relying on some chart magic which puts together a bundled secret from plain config values.

related #520 #493