Add Unit Type to Support Void Methods and requests for non-generic Result types

[ardalis]

MediatR has the same problems with returning things from void methods, and they've solved for years by using a Unit type: https://github.com/jbogard/MediatR/blob/master/src/MediatR.Contracts/Unit.cs

Should we use a similar type included in this library for this purpose, and document its usage?

See related issues:

• #77
• #65
• #41

What do you think? Comment or add an appropriate emoji.