nomination-pools: handle changes to `MinCreateBond`

[kianenigma]

this value should reflect how much the depositor puts in at the beginning. The question is, how to deal with it if this value changes.

If the value decreases, the depositor can unbond the difference. A test should demonstrate this, if already there isn't one.

If the value increases, we need a way to nudge the depositor into topping up the pool. A permissionless transaction can call bond_extra on the depositor, only with the delta of the current depositor's share and new minCreateBond.

I am not sure if this is worthwhile.

[polkadotcurry]

@kianenigma @rossbulat Given the fact that pools require a revisit to the client to rebond or withdraw their rewards, I believe we can assume reasonable visibility of a notice / banner placed at this moment. It could simlply reflect the diff between their bond amount and minCreateBond and the action(s) they can take.

It is not a proactive (nudge) solution to help realize the change as soon as it happens but I think it does address potential confusion by informing them at their next natural engagement.

I assume they can (1) adjust bond (2) administer pool (3) or just understand why they are not getting rewards and passively ignore action.

Thoughts?

[kianenigma]

I think a banner is great idea here, and UIs should implement it.

Moreover, on the protocol side, we can incentivize this further by allowing anyone to chill a pool, if the depositor's share in the pool is less than that of minCreateBond.

[kianenigma]

• force_unbond that unbonds you, and if needed, chills you.
• improve errors when a member fails to unbond their share from the pool, because the operator does not have enough skin in the game.
• UIs should