Minimal APIs endpoints require explicitly converting `Async<'T>` to `Task<'T>`

[brianrourkeboll]

Code like the following currently requires an explicit call to Async.StartAsTask in order for the asynchronous operation to be awaited by the framework, and for the correct response type metadata to be inferred:

FSharp.AspNetCore.WebAppBuilder/Examples/MinimalWebApp/Program.fs

Lines 58 to 74 in 21d4afa

	get "/api/clowns/{id}" [
	Status200OK, typeof<Dtos.Get.Clown>
	Status404NotFound, null
	Status500InternalServerError, typeof<ProblemDetails>
	] (fun (logger : ILogger<Program>) (db : IDataAccess) id ->
	db.Get id
	|> AsyncResult.teeError (function
	| Get.One.NotFound -> ()
	| Get.One.DbExn e -> logger.LogError ("A database exception occurred when trying to get the clown.", e)
	| Get.One.DbTimeout e -> logger.LogWarning ("A database timeout occurred when trying to get the clown.", e))
	|> AsyncResult.foldResult
	(Get.Clown.toDto >> Results.Ok)
	(function
	| Get.One.NotFound -> Results.NotFound ()
	| Get.One.DbExn _ -> Results.Problem "A database exception occurred."
	| Get.One.DbTimeout _ -> Results.Problem "A database timeout occurred.")
	|> Async.StartAsTask)

In contrast, the equivalent code in a controller action method does not require the explicit conversion:

FSharp.AspNetCore.WebAppBuilder/Examples/MinimalWebAppWithControllers/Controllers/ClownsController.fs

Lines 40 to 55 in 21d4afa

	[<HttpGet("{id}")>]
	[<ProducesResponseType(typeof<Dtos.Get.Clown>, Status200OK)>]
	[<ProducesResponseType(Status404NotFound)>]
	[<ProducesResponseType(typeof<ProblemDetails>, Status500InternalServerError)>]
	member _.GetClown id =
	dataAccess.Get id
	|> AsyncResult.teeError (function
	| Get.One.NotFound -> ()
	| Get.One.DbExn e -> logger.LogError ("A database exception occurred when trying to get the clown.", e)
	| Get.One.DbTimeout e -> logger.LogWarning ("A database timeout occurred when trying to get the clown.", e))
	|> AsyncResult.foldResult
	(Get.Clown.toDto >> Results.Ok)
	(function
	| Get.One.NotFound -> Results.NotFound ()
	| Get.One.DbExn _ -> Results.Problem "A database exception occurred."
	| Get.One.DbTimeout _ -> Results.Problem "A database timeout occurred.")

dotnet/aspnetcore#46898 will address this, if it is merged.