Add a model-skeleton for the HALight protocol

[thomaseizinger]

Problem

In order to execute Lightning swaps, our software needs to model the protocol that is described in the according spike: https://github.com/comit-network/spikes/blob/lnd-plan-of-attack/0022-lnd-plan-of-attack.adoc

Goal

Define a skeleton around the core concepts of the protocol:

• Ledger states
• Events
• Lightning ledger

Recommendation

Inspire yourself from the existing protocols: HAN and HErc20.
This is only about implementing a skeleton, hence having unimplemented!() in several places is totally fine!

Implementing the event traits (analogous to HtlcDeployed for HErc20 for example) of HALight, will uncover, what the API of a future LN connector is.
It is within the scope of this ticket to define a stubbed LndConnector that already has the API needed for our event traits.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[D4nte]

Done as part of #2297.