Implement changes for sub-workflow coordination

Please implement the design in https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/workflow-coordination.html (see !1079 (merged) and !1084 (merged)). We need the following changes:

• debusine:promise bare data: add category name to debusine.artifacts.models.BareDataCategory, and add a pydantic model for it (we don't have any models for bare data yet, but I guess somewhere in debusine.artifacts.models would do; it will need extra = pydantic.Extra.allow in its configuration, plus a root validator to forbid extra names starting with promise_)
• arrange to validate bare data in collections against pydantic models if available; I guess this could go in CollectionItemManager.create_from_bare_data, maybe?
• change WorkflowInternalManager.do_add_bare_data to forbid names starting with promise_ unless the category is debusine:promise
• change WorkflowInternalManager.do_add_artifact to copy variables into per-item data, and to forbid names starting with promise_
• change WorkflowInternalManager.do_add_artifact to allow an artifact to replace either bare data or another artifact (see DebianPackageBuildLogsManager.do_add_artifact), so that a real artifact can replace a promise
• change WorkRequest._retry_supersede to update the work request ID in any promises associated with the work request being retried
• change the scheduler to only call populate_workflow for root workflows, not sub-workflows (we'll work out the exact way that root workflows populate sub-workflows once we're implementing a real example of such a thing)
• add Workflow or WorkRequest helper methods to declare that a work request provides output / requires input (I'm not sure exactly what their signatures should be; let me know if you need more help defining these)
• change SbuildWorkflow to make each of its sbuild task children provide build-{architecture} output

The last two tasks here should be done together to make sure that the helper methods have useful interfaces, but the others are mostly independent.