Campaigns: API improvements of the promotion engine

Description

The default implementation of IPromotionEngine is made more modular, by splitting its logic into multiple components. It also supports custom settings to control the behavior of its Run method. You can control

• if rewards should be applied, or if it should just be a "dry run"
• statuses for which you want reward descriptions returned

Existing classes used by the promotion engine, such as the reward applicators, may also have been changed.

These changes should have minor effects on existing promotions and processors. But anyone who has made a custom IPromotionEngine, either a new implementation or a modification of the default one, should understand that these changes may have bigger implications.