Put IRequestHandler and INotificationHandler into MediatR.Contracts

[jpa67]

Hi,

if we had IRequestHandler and INotificationHandler in MediatR.Contracts,
we could define a handler in a .net standard library.

That could help a lot in isolating a domain layer in a DDD/Clean/Onion architecture.

best Regards

[BazInTheAZ]

This has been discussed previously here: #693

[jpa67]

Yes but I'm not agree with the reason to not doing it ;)

Then for now I will use my own version with this modification...