[billing] Retrieve user understandable pricing information from stripe

[svenefftinge]

Currently, the different prices are hard-coded per entity type (team/user) in the UI, this doesn't work if we change prices over time.
The source of truth is in stripe, so we should introduce a function in the billing service that allows retrieving the price for a cost center. This information should be retrievable for non-subscribed cost centers as well, so we can display what a user/team would sign up to.