Refactors the OnStakingUpdate trait

[gpestana]

This PR refactors the OnStakingUpdate trait to make its usage simpler and less error prone in the context of pallet-staking and the pallet-stake-tracker. The main goal is to make readily available more information/data required by the OnStakingUpdate listener (e.g. new stake on on_stake_update) through the interface methods.

With this refactor, the event emitter needs to explicitly call the events with more information about the event. The advantage is that this new interface design prevents potential issues with data sync, e.g. the event emitter does not necessarily need to update the staking state before emitting the events and the OnStakingUpdate implementor does not need to rely as much on the staking interface, making the interface less error prone.

Examples

Currently, on the OnStakingUpdate implemetor, we have

impl OnStakingUpdate<AccountId, Balance> for X {

  // snip..
 
  // the implementor will most likely need to fetch the nominations of the new nominator.
  fn on_nominator_add(_who: &AccountId) {

    // if the event emitter did not update the nominations at this point, the data is out of sync.
    let nominations = stakingInterface::get_nominations(); 
  }
}

with the new interface design, the implementor will have all the most important information implicitly in the method interface:

impl OnStakingUpdate<AccountId, Balance> for X {

  // snip..

  fn on_nominator_add(_who: &AccountId, nominations: Vec<AccountId>) {
    // no need to call into staking interface or other storage to fetch the nominations of the new nominator.
  }
}

Although the new design is less error prone, it is more opinionated and the design is highly affected by how the staking pallet will use this interface. Evidently, it is not possible to cover all the inputs that all the OnStakingUpdate implementors may need (depending on the use case), but the refactor aims at striking a good balance between safety for the usage in the context of staking and generality.

[gpestana]

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[rossbulat]

LGTM, useful additions.

[Ank4n]

Wouldn't this also need old and new self stake params?

[Ank4n]

Should we have both _prev_stake and _stake as same type? Either Stake<Balance> or Option<Stake<Balance>>. If Stake<Balance>, we can pass a zero value.

[gpestana]

Closing as these changes have been included in the stake tracker PR #1933 and only make sense in that context.