Add option to enable off-chain worker in SubxtClientConfig

[sander2]

To enable off-chain workers in integration tests, you need to manually edit the parachain config, as such:

let mut service_config = config.into_service_config();
service_config.offchain_worker.enabled = true;
let (task_manager, rpc_handlers) = btc_parachain_service::new_full(service_config).unwrap();
SubxtClient::new(task_manager, rpc_handlers);

It would be convenient if SubxtClientConfig contained an option to enable off-chain workers, so that you can use the normal SubxtClient::from_config instead of the above. It's honestly not a super big deal, but it'd help a lot in discoverability.

[ascjones]

client has been removed as part of #294