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.
Sorry I didn't explain myself. It is not a naming issue. I just don't get the purpose of the class. Most classes have construction parameters, but it is not a normal practice to group them all together into a single class and have that constructor accept that class. In this case the situation is worse, since there are parameters that aren't part of the parameters class. Maybe it is worth taking this offline since to avoid lengthy comments in github.