Add creation source to pipeline API and webhook

This MR is a reopen of gitlab-foss gitlab-foss!32504 (closed)

What does this MR do?

This MR adds a field named "reason" to the pipelines api response and webhook payload. It's possible values are those of the CI variable CI_PIPELINE_SOURCE.

"reason" is used as "source" would break unit-tests, and is also used for other purposes in api/webhook.

cf. #22352 (closed)

This MR only solves the API+Webhook part of the problem, which was my issue in #66771.

I did not touch the UI part at all (nor plan to).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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