create API to expose primary shelf config to frontend

[eviljeff]

all the admin model data needs to be exposed in a single API endpoint for frontend to consume

[AlexandraMoga]

@eviljeff I tried to figure out what can be tested at this point, but I can't find much.
I've noticed the new /hero/ endpoint but apart form verifying that the api calls are working, there isn't any data to validate.
I've also tried to generate data from the new "Hero" admin model but this tool doesn't seem to be finished yet.

If there is anything else that I could verify at this point please let me know.

[eviljeff]

It will be easier when #6813 is complete, yes, but it should be possible now to create a PrimaryHero instance, link it to a DiscoveryAddon and set some details. Nothing is validated though, and the details for the DiscoveryItem are only viewable/editable via the discovery admin at this point.

It won't be used on production for quite a while so there's no urgency to verify.

[AlexandraMoga]

Verified fixed on -dev
Both endpoints are returning the add-on details with the hero customization added in Discovery items.
GET /api/v4/hero/ - brings only one add-on randomly selected from the pool of Enabled primary heroes
GET /api/v4/hero/primary - brings the list of all Primary heroes enabled at a given moment

*Note: the description field will return the Custom description string added for the discovery item whenever this information is available. Otherwise, description will default to the add-on summary.