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.
That would mean convert `Amounts` into a path-dependent type. And therefore it cannot be used without a exchange object. And `Exchange` needs a `Amounts` object to be injected, or else the independent members should be passed in its construction to compose the `Amounts` object, making it useless.