Reward validators for participating in parachains (#2089)

* plumbing for rewarding backers

* give validators reward points for participating

* fix tests

* add bitfield rewarding

* add mocks for backing rewards

* add testing for backing & availability rewards

* implement RewardValidators on top of staking

* add to test-runtime and rococo

* add to test-runtime & rococo

* point to source on rewards values

* fix common tests

* do not reward availability anymore