Support internal API secret

What does this MR do?

This MR is based on !47 (merged)

The internal API secret, formerly known as Shell secret, is for Gitaly and other services to be able to call GitLab APIs. As a side effect, all applications that reuses the gitlab config will require the secret mounted into their fs.

• add internal api secret parameter to GitLab CR
• mount the secret into application, migration, job processor
• update the application config
• update the dev kit example CR

Author's Checklist

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Ensure a release milestone is set.
• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline on GitLab.com.
• When ready for review, MR is labeled workflowready for review per the MR workflow.

Expected

• Test plan indicating conditions for success has been posted and passes.
  • Follow the instructions in !46 (merged)
  • Can upload files to a repo
• Documentation is created or updated.
• Tests are added.

Related issues

• Closes #49 (closed)
• Blocker of #9