Implement passing trigger payload into pipeline variable [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR implements passing trigger payloads into pipelines as a variable (TRIGGER_PAYLOAD) with the type of file.

It will be used especially for webhook payloads.

Related to #31197 (closed)

It is behind a feature flag ci_trigger_payload_into_pipeline (#321027 (closed)).

Screenshots (strongly suggested)

Example CI config:

test:
  stage: test
  script:
    - echo $TRIGGER_PAYLOAD
    - cat $TRIGGER_PAYLOAD

Example trigger request:

And the result:

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