Allow loading multiple yaml documents from a single file

[NRUB]

Clear and concise description of the problem

It would be nice if we could define all our pipelines in a single .woodpecker.yaml file, instead of resorting to holding multiple pipelines in individual files in a dedicated folder. YAML syntax allows for multiple documents to be contained in a single stream by separating them with '---' (triple hyphens). Woodpecker currently doesn't utilized this feature when loading pipelines, since it's the file URI that is passed to - for example - 'genItemForWorkflow' that produces a single item from a single file, even if that file contains multiple documents.

Suggested solution

Allow for including multiple YAML documents within a single file.

This would require a change in some interfaces that currently take a file URI and pass it down to be processed by specialized functions first and extracting a list of YAML documents.

Alternative

No response

Additional context

Here are two relevant links to YAML official specification
2.2 Structures
4.3.1 Document

Validations

• Checked that the feature isn't part of the next version already [https://woodpecker-ci.org/versions]
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.