[Nomination Pool] Make staking restrictions configurable

[Ank4n]

closes #5742

Need to be backported to stable2503 release.

With the migration of staking accounts to fungible currency, we can now allow pool users to stake directly and vice versa. This update introduces a configurable filter mechanism to determine which accounts can join a nomination pool.

Example Usage

1. Allow any account to join a pool

To permit all accounts to join a nomination pool, use the Nothing filter:

impl pallet_nomination_pools::Config for Runtime {
    ...
    type Filter = Nothing;
}

2. Restrict direct stakers from joining a pool

To prevent direct stakers from joining a nomination pool, use pallet_staking::AllStakers:

impl pallet_nomination_pools::Config for Runtime {
    ...
    type Filter = pallet_staking::AllStakers<Runtime>;
}

3. Define a custom filter

For more granular control, you can define a custom filter:

struct MyCustomFilter<T: Config>(core::marker::PhantomData<T>);

impl<T: Config> Contains<T::AccountId> for MyCustomFilter<T> {
    fn contains(account: &T::AccountId) -> bool {
        todo!("Implement custom logic. Return `false` to allow the account to join a pool.")
    }
}

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/13550269016
Failed job name: fmt

[joepetrowski]

I think I would name this something like AllDirectStakers or AllNativeStakers, since people who join nomination pools could still be considered "stakers" (i.e. they have their funds at stake). This is checking that they have an entry in the staking ledger specific to their account.

[paritytech-cmd-bot-polkadot-sdk]

Created backport PR for stable2407:

• [stable2407] Backport #7685 #7736 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-7685-to-stable2407
git worktree add --checkout .worktree/backport-7685-to-stable2407 backport-7685-to-stable2407
cd .worktree/backport-7685-to-stable2407
git reset --hard HEAD^
git cherry-pick -x f7e98b40cab7475898c99ea48809635ac069af3a
git push --force-with-lease

[paritytech-cmd-bot-polkadot-sdk]

Created backport PR for stable2409:

• [stable2409] Backport #7685 #7737 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-7685-to-stable2409
git worktree add --checkout .worktree/backport-7685-to-stable2409 backport-7685-to-stable2409
cd .worktree/backport-7685-to-stable2409
git reset --hard HEAD^
git cherry-pick -x f7e98b40cab7475898c99ea48809635ac069af3a
git push --force-with-lease

[paritytech-cmd-bot-polkadot-sdk]

Created backport PR for stable2412:

• [stable2412] Backport #7685 #7738 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-7685-to-stable2412
git worktree add --checkout .worktree/backport-7685-to-stable2412 backport-7685-to-stable2412
cd .worktree/backport-7685-to-stable2412
git reset --hard HEAD^
git cherry-pick -x f7e98b40cab7475898c99ea48809635ac069af3a
git push --force-with-lease

[paritytech-cmd-bot-polkadot-sdk]

Successfully created backport PR for stable2503:

• [stable2503] Backport #7685 #7739