Runtime for testing Bitcoin smart contracts

[nud3l]

Is your feature request related to a problem? Please describe.
A key use case for building smart contracts on Interlay will be the ability to read trustlessly from Bitcoin via the BTC light client.

Developing Bitcoin smart contracts requires access to at least a basic runtime with the following functionalities deployed in addition to essential runtime pallets (tokens, ...).

• Contracts pallet
• BTC-relay
• Sudo

Spinning up a test environment for Interlay is quite involved (https://github.com/interlay/interbtc-api/blob/master/docker-compose.yml).

I would like to simplify the effort to setup a development environment that provides access to the functionality listed above.

Describe the solution you'd like
Move the BTC relayer functionality to an off-chain worker so that anyone can setup a interlay or kintsugi runtime has to start a local substrate node. They can then chose to relay Bitcoin testnet blocks or run a local Bitcoin regtest network.

[sander2]

Started on this task. I'm trying to add the relay to the node service. The reason for not going the offchain worker is that the offchain worker is executed in wasm (no_std), which makes interfacing with the bitcoin daemon more complicated.

There are two possible approaches when going the service route: we can either use the jsonrpc interface, or try to submit the extrinsics directly from the node internals. I'm trying the latter option because I'd like to avoid a subxt+metadata dependency if possible and I've found a way to construct and submit extrinsics although it's still untested atm