Make `pallet-bridge-rewards` generic over `RewardKind` (#7492)

Closes: https://github.com/paritytech/polkadot-sdk/issues/7272
Relates to: https://github.com/paritytech/polkadot-sdk/pull/6578
Relates to: https://github.com/paritytech/polkadot-sdk/issues/7274

## Description

The PR enhances the `pallet-bridge-rewards` by making it generic over
the `RewardKind` type (previously hardcoded as `RewardsAccountParams`).
This modification allows the pallet to support multiple reward types
(e.g., P/K bridge, Snowbridge), increasing its flexibility and
applicability across various bridge scenarios.

Other pallets can register rewards using `bp_relayers::RewardLedger`,
which is implemented by the rewards pallet. The runtime can then be
configured with different mechanisms for paying/claiming rewards via
`bp_relayers::PaymentProcedure` (e.g., see the `pub struct
BridgeRewardPayer;` implementation for BridgeHubWestend).

## Follow-up  
The removed balances/rewards statistics from the complex relay (no
longer used) will eventually be reintroduced or fixed in the standalone
relayers via
https://github.com/paritytech/parity-bridges-common/issues/3004#issuecomment-2401634589.

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Serban Iorga <serban@parity.io>