Dynamic offers: hopefully functional!

[Hunter Freyer]

Hey Drivers folks!

fxrev.dev/547027 has just landed, meaning dynamic offers should be usable! 

Steps to use:

1. In the `fuchsia_component` BUILD rule for the component containing the collection that uses dynamic offers, add `restricted_features = [ "dynamic_offers" ]`. This will require updating this allowlist. 
   
2. In CML, set `allowed_offers: "static_and_dynamic"` on the collection in which the dynamic components will be created.
   
3. When creating the dynamic child, set CreateChildArgs.dynamic_offers.

Caveats/TODOs:

1. Currently, you can create a dynamic offer whose source doesn't exist. This will become an error in a subsequent CL, so don't do that!
   
2. Dynamic offers are not yet taken into account for component shutdown order. This could theoretically cause flakiness while the system shuts down, but in the drivers case I doubt it will.

Please assign me any bugs you discover! 

Thanks,

Hunter

[Abdulla Kamar]

Yay, thank you! \o/

I'll start plumbing this through the driver framework late this week, or early next week.