Do not require internal license checkers to be registered in the container

[jbogard]

See #1126

The internal license checkers are resolved by the container instead of instantiated directly. This causes problems when users don't use IServiceCollection. The ServiceRegistrar exposes registration methods but it's still required to use IServiceCollection.

Instead, we can only require MediatRServiceConfiguration to be registered (where all the configuration is), which is public.

[alsami]

Using MediatRServiceConfiguration is also not an option here, since in my case, I do not use that type. If there is no way to make the liscence types public, it is fine for me to stick to reflection to register them.

#1125

[jbogard]

Using MediatRServiceConfiguration would be the option because you'd then register that type.