[Pools] New runtime api that returns the pot accounts associated with the pool

[Ank4n]

closes #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

[Ank4n]

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[rossbulat]

Excellent, this will help applications a lot 🫡

[kianenigma]

This is good to add, but we will probably migrate it to #4722 🙈

we can keep the existing APIs for backwards compatibility for some time.