Contracts (ink!) support

Is your feature request related to a problem? Please describe. Developing smart contracts with Bitcoin capabilities relies on using Solidity and the EVM mostly. We can offer developers Rust contracts with ink! using the contracts pallet and giving it access to the BTC-relay.

Describe the solution you'd like Implement a proof of concept of ink! Bitcoin contracts.

[ ] Add the contracts pallet to the Kintsugi and Interlay runtimes
[ ] Give the contracts pallet access to the BTC relay functions (verify transactions)
[ ] Implement a POC smart contract that verifies a Bitcoin transaction.

Describe alternatives you've considered https://github.com/interlay/interbtc/issues/1098

Additional context

Maybe, we should address #605 first to not break the interface directly.

interlay / interbtc

Contracts (ink!) support #983