Introduce untested domain model for the exchange protocol

### Description

Introduce a domain model for the exchange protocol. The model will be able to handle all the transactions and validations needed for it encapsulated so the rest of the codebase is agnostic about how this is done.
The contributed model is partially implemented and the behavior will be migrated from `DefaultHandshake`, `DefaultExchange` and other parts of the code to this package in a series of PRs. In this first one, the multisig creation code is moved to this package.

### Testing

Existing tests pass.

### Coverage

Unchanged for the existing code.
Confusing... Does calling the close method actually closes the payment channel? It doesn't seem like it, but the method signature signals that it has side effects (`close()` vs `close`). This is probably related to the `Closing` name, which I also think is confusing.