New Multi-Form Entity

CiviEvent is limited because there is only the option to have one registration form per event.
We could add an entity to the schema that will enable collecting multiple uf_groups into a single user-facing "form". Call it entity_form.

This entity will enable entries in the uf_join table that reference this entity.
The entity_form record will have a parent_entity, that in CiviVolunteer's case will be a volunteer project.

Due to general applicability, this should be created as its own extension.

rough schema:

• id
• parent_enity_type
• parent_entity_id
• title
• Header
• Footer
• serialized data