Support ValueTask as asynchronous return type from functions

It appears that [DefaultMethodInvokerFactory](https://github.com/Azure/azure-functions-dotnet-worker/blob/main/src/DotNetWorker.Core/Invocation/DefaultMethodInvokerFactory.cs#L159-L178) only treats `Task` and `Task` as method return types that need to be `await`ed. This means that if you use `ValueTask` or `ValueTask`, you'll end up dealing with strange exceptions as your code will run to the first `await` and then the DI scope will be disposed, including all injected dependencies like DbContexts.

Personally, I'm a fan of [preferring ValueTask to Task](https://blog.marcgravell.com/2019/08/prefer-valuetask-to-task-always-and.html) so not being able to use `ValueTask` was a bit confusing at first, but we are able to work around this in the meantime by using `Task`.

Start in src/DotNetWorker.Core/Invocation/DefaultMethodInvokerFactory.cs at lines 159-178 and inspect how asynchronous return types are identified and awaited. Verify the behavior for both ValueTask and ValueTask, including whether injected dependencies remain available until the function completes.

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