Add support for manual bridges for CI pipelines

What does this MR do?

Related to: #201938 (closed)

This MR adds the support of using "when: manual" with bridges. With this, you can use "when: manual" with "trigger" keyword for a job.

This feature is behind a feature flag ci_manual_bridges #263412 (closed).

Screenshots

.gitlab-ci.yml

build:
  stage: build
  script: echo build

test:
  stage: test
  when: manual
  trigger:
    include: '.child-pipeline.yml'

deploy:
  stage: deploy
  script: echo deploy

.child-pipeline.yml

build:
  stage: build
  script: echo build

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