Move EraIndex to sp-staking (#10671)

* Move `EraIndex` to sp-staking

* Fix imports in mocks

* Run cargo +nightly-2021-10-29 fmt

substrate/primitives/staking/src/lib.rs

@@ -24,3 +24,6 @@ pub mod offence;
 
 /// Simple index type with which we can count sessions.
 pub type SessionIndex = u32;
+
+/// Counter for the number of eras that have passed.
+pub type EraIndex = u32;