Improve CQS System

[yceruto]

This proposal contains several improvements:

• Using generic templates in Command/Query classes, so documenting the returned type for each dispatch/ask invocation is no longer necessary.
• Now FindBookQuery strictly returns a Book instance; otherwise, a MissingBookException is thrown.
• The MissingBookException is mapped to return a 404 status code, so a clear exception message is now displayed.
• Replace CommandHandlerInterface with the AsCommandHandler attribute, and QueryHandlerInterface with the AsQueryHandler attribute. The reason behind this is the fact that these interfaces have no contract; they are used as metadata to configure the messenger handler only. So, in this case, we can now use attributes instead as they are intended for that purpose.

[mtarld]

Thank you, @yceruto, that's a great code simplification 🙂

[chalasr]

Thanks Yonel!

[yceruto]

Thank you guys for this demo!