Staking rate targeting and specific rewards. (#2882)

* PNPoS implementation

* wip: change staking api

* code readibility

* fix overflow

* comment

* license

* doc

* reorganize a bit

* rename to proper english + doc

* address comments

* refactor unused mock

* fix add_point

* update tests

* add not equalize to ci

* Revert "add not equalize to ci"

This reverts commit 1c61c25874b04b1cdfadd7d353b75a189a59ad35.

* bring test back

* update locks

* fix genesis config

* add authorship event handler + test

* uncouple timestamp from staking

* use on finalize instead

* remove todo

* simplify mock

* address comment

* doc

* merge test

* fmt

* remove todo todo

* move add_reward_point to regular function

* doc

* doc

* increase version

* doc and fmt

* Update srml/staking/src/inflation.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Fix some doc typos