Improve ApiVersioning support by automatically generate routes using Controllers definitions

[OculiViridi]

Following #1393 and #1355, I'm only suggesting a possible approach for a better API Versioning support.

It would be really useful and less mistake-prone if NSwag could handle the different decleared versions of an API automatically.
Actually, by following #1393, all the different versions of API needs to be registered and configure manually in Startup (version number matching the one in the Controller Attribute, files, etc.).
A possible different approach could be to have something like this:

app.UseSwaggerUi3(typeof(Startup).GetTypeInfo().Assembly, config =>
{
	/* Looks inside Controllers for all ApiVersion() attributes and generates a route and a JSON file for each one.
	 * Also the DefaultUrlTemplate property could be taken from Route() controller attribute, if present.
	 */
	config.GeneratorSettings.UseMultipleVersions = true;
	config.SwaggerRoutes.LoadFromControllers();

	// Generic settings for all routes
	config.GeneratorSettings.Title = "My Amazing API";
	config.GeneratorSettings.Description = "Amazing API functionalities.";
	
	// Specific settings for single route
	foreach (var route in config.SwaggerRoutes)
	{
		route.DefaultUrlTemplate = "{v:apiVersion}/{controller}/{action}/{id?}";
		route.DefaultPropertyNameHandling = PropertyNameHandling.CamelCase;
	}
});

Thank you!

[RicoSuter]

For reference, here are the existing tests: https://github.com/RSuter/NSwag/blob/master/src/NSwag.SwaggerGeneration.AspNetCore.Tests/ApiVersionProcessorWithAspNetCoreTests.cs

Controllers:
https://github.com/RSuter/NSwag/tree/master/src/NSwag.SwaggerGeneration.AspNetCore.Tests.Web/Controllers

[alexsandro-xpt]

Some news about that?