Show merged version of ci config [RUN AS-IF-FOSS] [RUN ALL RSPEC]

What does this MR do?

This adds a tab that shows the mergedYaml value of a CI config inside the pipeline Editor section. It also adds some additional logic to hide the commit form when in the merged tab to avoid confusion for the user about what their file looks like when committing.

Behind a disabled feature flag: ci_config_merged_tab

Screenshots (strongly suggested)

Before	After

Without errors

With errors

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