[Pools] New runtime api that returns the pot accounts associated with the pool (#6357)

closes https://github.com/paritytech/polkadot-sdk/issues/6358

Adds the following runtime api to pallet-nomination-pools.
`pool_accounts(pool_id)`: Returns `(bonded_account, reward_account)`
associated with the `pool_id`.

cc: @rossbulat

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Co-authored-by: command-bot <>