AddOpenBehavior(type) now supports stream behaviors without duplicate registrations

[asimmon]

What's changed:

• MediatRServiceConfiguration.AddOpenBehavior(type) assumed that the type parameter implemented IPipelineBehavior<,>. It now supports IStreamPipelineBehavior<,> and throws an InvalidOperationException the type parameter is not actually an open behavior. Added multiple tests.

• TryAddTransientExact was replaced by the built-in IServiceCollection.TryAddEnumerable that does the exact same thing, i.e. skip a registration if there's already a service descriptor that exactly match the service type AND implementation type. Modified an existing test to prove that everything behaves as before.

• MediatRServiceConfiguration.BehaviorsToRegister consumption now use TryAddEnumerable too. Covered by the modified test from the previous point.

• Apply the 11.x to 12.x migration guide in test classes: several test open behaviors were not using the right generic constraint, which is now TRequest : notnull.

[asimmon]

Hi @jbogard would you consider releasing these changes in a new patch?