PoC: Selectable CI Templates

What does this MR do?

• Adoptable template
  • Organize the CI template structure
    • <Context>/<Type>/<Name>.gitlab-ci.yml
    • Context can be programming language or frame work.
    • When the language used in repository matches <Context>, a corresponding template is suggested. e.g. Ruby 67.9%, JavaScript 19.6%, etc.
    • Type can be one of pipeline, stage or job
      • pipeline type is the template to create an end-to-end CI/CD workflow.
      • stage type is the template to create a specific stage in CI/CD workflow.
      • job type is the template to create a specific job in CI/CD workflow.
    • Global keys can be defined in Global context.
  • Users can adopt specific pipeline/stage/job at will.
• Programmable template
  • YAML-Template Renderer Jinja2 - https://ttl255.com/jinja2-tutorial-part-1-introduction-and-variable-substitution/
  • We can percentage-rollout newer template with breaking change
  • We can allow new projects to use newer template
  • We don't need latest version template.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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:

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