Unify bounds for the maximum number of validators across pallets

[re-gius]

Currently, there are several constants or storage items bounding the maximum number of validators:

• MaxValidatorsCount storage item from pallet staking (optional for now, but will be mandatory with Add storage bounds for pallet staking and clean up deprecated non paged exposure storages #6445 )
• MaxAuthorities from pallets babe and grandpa
• MaxActiveValidators from pallet election-provider-multi-phase (sometimes equal to MaxWinners)

This may introduce incoherent behavior in benchmarking and tests.

Proposed solution
@kianenigma suggested here to expose integrity checks as a part of a trait SessionManager.

This trait can expose a trat SessionManager { type MaxAuthorities = }. Then, within the pallet-session, who consumes SessionManager, you can do:

fn integrity_check() { 
    /// A way to express within pallet-session that whoever implements `SessionManager` should have a compatible `MaxAuthorities`. 
    assert!(T::SessionManager::MaxAuthoritiet::get() >= T::MaxAuthorities::get())
}