Add [JsonConverter(typeof(StringEnumConverter))] to Enums outside of my Assembly

Hey,

I'm running into an issue with enums showing up as numbers in the swagger UI. According to the documentation I need to add `[JsonConverter(typeof(StringEnumConverter))]` to the enum file. However, for enums outside of my assembly or in Nuget packages I can not add that attribute. I tried doing it via reflection in the Azure Functions Startup.cs. see code below:
``` csharp
// add [JsonConverter(typeof(StringEnumConverter))]
var nugetAssembly = typeof(IBaseModel).Assembly;
var enumTypes = nugetAssembly.GetTypes().Where(t => t.IsEnum).ToList();
enumTypes.ForEach(e => TypeDescriptor.AddAttributes(e, new JsonConverterAttribute(e)));
```

I confirmed by printing out the Attributes that the JsonConverterAttribute is in the enum in that assembly, but the swagger documentation still shows them as ints instead of string.

If this is a known bug please let me know.

Thanks!

Start in the Azure Functions Startup.cs example and reproduce the external-enum case using the supplied TypeDescriptor.AddAttributes code. Trace how enum schemas are generated for NuGet assemblies and compare the result with the Swagger UI output. Done means externally defined enums appear as strings rather than integers in the generated OpenAPI documentation.

Written by the indexing model from the issue text.