T205444: Provide a way of having a meaningful slot header

Phabricator Link	T205444
Status	open
Priority	needs triage (violet)
Author	[1]
Owner
Projects	Multi-Content-Revisions (Tech Debt) CPT Initiatives (MCR) Structured Data Engineering (Monitoring) Structured-Data-Backlog (Tracking)
Subtasks	T194046: Introduce SlotRoleHandler and SlotRoleRegistry for declaring slot roles.
Parent Tasks	T200915: Allow SlotRoleHandlers to control page layout
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