[MCR] Introduce SlotRoleHandler and SlotRoleRegistry

These new classes provide a mechanism for defining the
behavior of slots, like the content models it supports.
This acts as an extension point for extensions that need
to define custom slots, like the MediaInfo extension
for the SDC project.

Bug: T194046
Change-Id: Ia20c98eee819293199e541be75b5521f6413bc2f