SwaggerToCSharpControllerGenerator: Generate Task<ActionResult<T>> for ASP.NET Core

Please add an ability to generate controller actions that return ActionResult<T>/Task<ActionResult<T>> for ASP.NET Core.

Ref: #1386

[optuuttila]

I actually played around this a bit and came up with this fork: https://github.com/optuuttila/NSwag/commit/cef9ecc1693bde731a636c52173562a832e3ef6a

It results in following kind of generated code: Interface: System.Threading.Tasks.Task<Microsoft.AspNetCore.Mvc.ActionResult<MyModel>> MyMethodAsync(string param1, string param2);

Controller: public System.Threading.Tasks.Task<Microsoft.AspNetCore.Mvc.ActionResult<MyModel>> MyMethod(string param1, string param2) { return _implementation.MyMethodAsync(param1, param2); } Obviously the code would need to be finalized (a PR for the NJsonSchema.CodeGeneration.CSharp package), but before doing any of that I just wanted to ask whether this is anything along the lines you would accept as an PR?

[altso]

I believe it’s the same as with System.Net.Http.HttpResponseMessage for ASP.NET WebAPI which is allowed right now (https://github.com/RSuter/NSwag/blob/master/src/NSwag.CodeGeneration.CSharp/Templates/Controller.liquid#L94).

It gives you a better flexibility around http status codes.

[HttpPut("resource/{id}")]
public async Task<ActionResult<Resource>> UpdateResource(int id, [FromBody] Resource resource)
{
    if (/* resource exists */)
    {
        if (/* custom validation (e.g. check for fields that could not be updated) */)
        {
            if (/* user is permitted to update this specific resource */)
            {
                // update resource and return an updated copy (e.g. last modified date field)
                var updatedResource = _resourceService.Update(resource);
                return Ok(updatedResource);
            }

            return Forbid();
        }

        return BadRequest();
    }

    return NotFound();
}