Fedimint support

In addition to LNDHubX support, we would like to offer an integration into Fedimint, as a federated custody Lightning solution.

For this, we will need to change the existing methods from `lightning` to `lndhub`, and ideally, we would keep many similar methods with the same name, so switching calls to either is simplified. The code may need to be reorganized, especially keeping in mind we would also like to support LDK and async payments in the future.