Create new Billing endpoints in the front end and backend

[anamarn]

Right now I use the same endpoints in the resolver and using an alternative path given the value of the feature flag. I would like to add new endpoints in the resolver that uses the notions of plan Key:

FrontEnd GQL:

• getProductPrices => billingBaseProduct (BillingPlan Output -> Base Product -> [Reccuring Interval, StripePriceId])
• upgradeOrDowngradeBillingPlan (change the stripe prices ids to the ones corresponding to the other plan, remember to update the subscription metadata in order to contain the plan Key)

Use the Plans query and select the correct fields in gql