Add Premium Product v4 api

[eileenmcnaughton]

No description provided.

[civibot]

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[eileenmcnaughton]

I added Premiums api in the hope it addresses this

(fun fact premiums is our only plural entity)

Test Result (1 failure / +1)
api\v4\Entity\ConformanceTest.testConformance with data set "PremiumsProduct"

[eileenmcnaughton]

sigh - this one is a bit of a fight - test fails on entity_id - I guess I need to add a list of availble to the Product entity - - I wonder what entities are possible

@colemanw I note that the tables are

civicrm_premiums
civicrm_premiums_product

Based on the BAO the entities are Premium and PremiumsProduct - it's bit inconsistent with the plurals but have just followed the BAO for now.