Opinionated handler design does not fit slim/wrapped handlers that only forward invocations

[kbaley]

This issue was transferred from a private repository

NServiceBus uses an opinionated handler design that sometimes is not worth the effort. Especially if handlers do not contain the business logic and are wrappers.

It requires assembly scanning while a simple callback registration API could in some cases be a better approach for our users.

There is already a community package called Community.NServiceBus.LambdaHandlers by @timbussmann that provides this functionality. It makes sense to have this registration based callback API available in Core.