Add support for `when:manual` within triggered pipelines

Problem to solve or address

Using trigger to create multi-project or parent-child pipelines does not support when: manual configs, with the docs stating "when:manual prevents triggers being used." The best reasoning I can find for this is a slight mention in a note in MR 9107:

we might not want to support delayed and manual yet

Customer requesting (internal only) the use of manual actions for parent-child pipelines. Their specific use-case here is to refactor .gitlab-ci.yml to separate out certain stages from an otherwise fairly complex CI/CD, and wanting to limit their deploy job to a manual action.

Is there a specific reason for limiting the use of when:manual, and if so, perhaps the docs for parent-child pipelines could be updated to reflect that? If not, was it ever an intention to implement that functionality?

It's technically possible to implement this using a parent job to trigger a child pipeline that then uses manual jobs, so there is a workaround to this feature request at present. But since trigger supports when (on_success, on_failure, and always), it is a valid question from the end-user perspective as to why manual is not currently supported.

Related docs improvement issue for trigger and when:manual.