CPC Campaigns

[Ivshti]

Is your feature request related to a problem? Please describe.
Some advertisers prefer the CPC payment model rather than CPM

Describe the solution you'd like
When creating a campaign, allow choosing CPC rather than CPM.

Additional context
Selecting CPC would work just the same way as CPM, but using pricingBounds we'll set the IMPRESSION prices to 0, and the CLICK price to a range. We'll use the same range-based system as for impressions, where publishers are grouped into groups by category and given a different price.

Depends on AmbireTech/adex-market#126