Draft: Allow jobs need to refer to a job in the same stage

What does this MR do?

Related to #297083 (closed) (spike issue). Original issue: #30632 (closed)

This MR is a POC of allowing jobs to need jobs in the same stage.

For a ready-MR, this requires;

• Handling circular dependencies.
• Handling .before_stage, .for_stage and .after_stage as this will break that, ex.: retry of build, dependencies, analytics
• UX / some designs.
• A feature flag
• documentation changes

Screenshots (strongly suggested)

Example 1

build:
  stage: test
  script: exit 0

test:
  stage: test
  script: exit 0
  needs: [build]

deploy:
  stage: test
  script: exit 0
  needs: [test]

Example 2

build1:
  stage: test
  script: exit 0

build2:
  stage: test
  script: exit 0

test1:
  stage: test
  script: exit 0
  needs: [build1]

test2:
  stage: test
  script: exit 0
  needs: [build2]

deploy1:
  stage: test
  script: exit 0
  needs: [test1]

deploy2:
  stage: test
  script: exit 0
  needs: [test2]

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