[Pools] Fix issues with member migration to `DelegateStake` (#4822)

## Context
Pool members using the old `TransferStake` strategy were able to
transfer all their funds to the pool. With `DelegateStake` changes, we
want to ensure similar behaviour by allowing members to delegate all
their stake to the pool.

## Changes
- Ensure all the balance including ED of an account can be delegated
(and used in the pool) by adding a provider for delegators.
- Gates calls that mutates the pool or pool member if they are in
unmigrated state. Closes
https://github.com/paritytech-secops/srlabs_findings/issues/409.
- Adds remote test to migrate all pools and members to `DelegateStake`
which can be used with `Kusama` and `Polkadot` runtime state. closes
https://github.com/paritytech/polkadot-sdk/issues/4629.
- Add new runtime apis to read pool and member balance.

## Addressing possible migration errors 
Pool members migrating can run into two types of errors:
- Already Staking: If the pool member is already staking, we cannot
migrate them to `DelegateStake` since this may mean they are able to use
the same staked funds in the pool. Users would need to withdraw all
their funds from staking, in order to migrate their pool funds.
- Pool contribution below ED: For these cases transfer from pool account
to member account would fail. The affected users can top up their
accounts and redo migration.

Another error that was earlier possible was when member's free balance
is below ED. This PR adds a provider to delegator allowing all user
balance including ED can be contributed towards the pool. This helps
`1095` accounts in Polkadot and `41` accounts in Kusama to migrate now
which would have earlier failed.

## Results from RemoteExternalities Tests.

### Kusama
`Migration stats: success: 3017, direct_stakers: 361, unexpected_errors:
0`

### Polkadot
`Migration stats: success: 42859, direct_stakers: 643,
unexpected_errors: 0`

## TODO
- [x] Add runtime api for member total balance.
- [x] New
[issue](https://github.com/paritytech/polkadot-sdk/issues/5009) to reap
pool members with contribution below ED.
- [x] Add provider for delegators so whole balance including ED can be
held while contributing to pools.
- [x] Gate all pool extrinsics if pool/member is in non-migrated state.

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Co-authored-by: Gonçalo Pestana <[email protected]>