Http trigger serializes returned ValueTask<IActionResult> as POCO

Http triggers allow you to return a `Task`, and the host will await the task and execute the result. However, if I return a `ValueTask` instead, the whole value task is serialized:

[FunctionName("Foo")]
public ValueTask ExecuteAsync([HttpTrigger(AuthorizationLevel.Function, "get")]HttpRequest request)
{
return new ValueTask(OkObjectResult("Foo"));
}

Expected output:

"Foo"

Actual output:

{
"isCompleted": false,
"isCompletedSuccessfully": false,
"isFaulted": false,
"isCanceled": false,
"result": {
"value": "Foo",
"formatters": [],
"contentTypes": [],
"declaredType": null,
"statusCode": 200
}
}

In most cases the return type of the function can simply be changed from `ValueTask` to `Task`. However it is not the behaviour I expected, and since there is already a check for `Task`, I imagine it wouldn't be hard to also check for `ValueTask`. I'm happy to implement this if I'm allowed.

- Microsoft.NET.Sdk.Functions 3.0.11

Start at the HTTP trigger return-value handling and locate the existing check for Task results. Reproduce the example with a ValueTask and compare it with the Task behavior. Done means the ValueTask is awaited and its IActionResult is executed so the response is "Foo" rather than serialized ValueTask state.

Written by the indexing model from the issue text.