Add readonly editor to display current CI config

What does this MR do?

Add readonly editor to visualize ci config

As a first iteration on the CI editor, this change adds an simple editor (editor lite) and loads data of a single .gitlab-ci file.

New page pipeline authoring behind feature flag :ci_pipeline_editor_page

TODO

• Implement using a state management (such as apollo)
• Load data using Graph QL
• [-] Check if other data (such as ref, default branch, etc...) is relevant at this stage.
  • The branch has to be considered later, but for now, we just use the default branch.

Screenshots (strongly suggested)

Sometimes the file is missing...

no branches exist	no file in repo

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

Related to #263141 (closed)