Add "UseTaskWhenAll" option

[fretje]

I am using this libaray in blazor, and due to the fact that the event handlers are run sequentially, it sometimes takes a while for components to update, because they have to wait for other components that subscribe to the same events.

I work around this now by changing most of my event handlers to void methods and use this pattern instead:

_ = InvokeAsync(async () => await ...);

But just like mediatr now has its TaskWhenAllPublisher, I thought I make something similar for Courier.

This pr is the result.

When the new option "UseTaskWhenAll" is true, all the tasks are now gathered into a list, and then awaited for at the end with Task.WhenAll, in stead of sequentially.

I have also added a couple of tests.

Let me know if the naming is ok. The option could be named something like "PublishParallel" as well?

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With this I would be able to use actual async event handlers again so I don't have to resort to the "InvokeAsync pattern" I talked about above.

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This is based on #19

[sonarqubecloud]

Please retry analysis of this Pull-Request directly on SonarQube Cloud

[KuraiAndras]

Please update to the latest master so I can see the changes only in this branch

[fretje]

Wait for the merge, I'm adding some info to the readme as well...