Pallet staking events to named enum (#12342)

* Pallet staking events to named enum

* fmt

* update np staking tests

* update remaining events

* update benchmarks

* Update frame/nomination-pools/test-staking/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/staking/src/pallet/mod.rs

* Update frame/staking/src/pallet/mod.rs

* Update frame/staking/src/lib.rs

* Update frame/staking/src/pallet/impls.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: parity-processbot <>
Co-authored-by: Ankan <10196091+Ank4n@users.noreply.github.com>