distribute wrapped fees as escrow reward

[gregdhill]

Signed-off-by: Gregory Hill [email protected]

The community have suggested that distributing bridge fees to stake-to-vote accounts would add additional utility to the native token (INTR / KINT). This PR changes the fee pallet to additionally include the EscrowRewards configuration and an additional extrinsic withdraw_escrow_rewards to yield all rewards for the given currency_id.

I've removed most of the nomination / reward tests since I'm not interested in maintaining unused code and added two new integration tests for generalized vault / escrow reward distribution and withdrawal.