Use parachain RPCs for complex functionality

[gregdhill]

Instead of re-implementing (and testing) complex parachain logic, we should switch to using RPCs.

• [x] Escrow - balanceAt + totalSupply
• [ ] Issue - request (find vaults for amount)
• [ ] Nomination
• [ ] Redeem - request (find vaults for amount)
• [x] Vaults - computeReward + getBlockRewardAPY, getPremiumRedeemVaults

See: https://github.com/interlay/interbtc/issues/695

[theodorebugnet]

Note that on Issue and Redeem, the lib implements extra lib-only functionality (namely, support for batching). I don't think we had any plans for implementing batching in the parachain directly, so these would not be replaceable by RPCs.