ORTB Blocking: Support defining bcat taxonomy

[bretg]

It's been requested that the PBS ORTB blocking module support defining cattax.

This is easy enough on the signalling side, but the enforcement will need community input.

1. add a blocked-adv-cat-tax directive, which maps to ORTB $.cattax. This should be supported on overrides as well.
2. When enforcing, verify that the bid response cattax matches the publisher-defined blocked-adv-cat-tax. Default value of blocked-adv-cat-tax is 1. If the response cattax doesn't match, reject the bid response.
3. No changes in seatnonbid, metrics, or logging.

[bretg]

Discussed in committee. We're going to defer any changes here until the cattax-related discussions in the IAB TechLab are resolved.