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.
I think these abstract types are going to be problematic. We can keep them and see how that goes, but the compiler has a tough time proving that two instances come from the same abstract type.