Automate the update logic for MinNominatorBond and MinValidatorBond

[kianenigma]

Kinda similar to the work done in https://github.com/paritytech/substrate/pull/12970/files#, we want to be able to have possibly automatic logic updating these values. A default implementation could still easily set them to value stored onchain.

Multiple implementation options exist (taking the example of updating MinNominatorBond only):

1. Custom for staking. Similar to how [NPoS] Implements dynamic number of nominators substrate#12970 introduced a new trait NominationQuota, we also add new custom traits, bake the logic deep into the staking pallet. Staking will decide when and how to update this. A configuration item will just decide which update mode is being used.

2. Use a type Convert<InputBalance, OutputBalance> to receive the new value. InputBalance would be the MinActiveStake added in store min-active-bond onchain substrate#12746, output would be the new MinNominatorBond.

3. Use a slightly more generic type Get<Balance>. called per era, it should return the new MinNominatorBond.

Probably more ways to do this.

Inspired by the comment in https://polkadot.polkassembly.io/post/1650

[MrishoLukamba]

Am taking it @kianenigma