[NPos] Expose pallet-delegated-staking via extrinsics

[Ank4n]

This would allow a future smart contracts (for example when staking moves to Asset Hub) to build their own implementations of pool. It would require careful review and refactor of current code to keep it secure.

Non exhaustive list of issues:

• Take deposit when registering an agent.
• Ensure agent accounts are keyless and generated by the pallet. (so direct operations on staking pallet cannot be done via these).