Allow specifying variables when running manual jobs

What does this MR do?

EE MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14784

• Backend
  • Write Job variable application code
  • Write Job variables database migration
  • Write Job variable specs
• Frontend
  • Create component with form
  • Render the manual variables component when playable only
  • Handle secret value change
  • POST request
  • Update the text
  • Add specs

Does this MR meet the acceptance criteria?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/24935

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• N/A Label as security and @ mention @gitlab-com/gl-security/appsec
• N/A The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• N/A Security reports checked/validated by a reviewer from the AppSec team