ListResourcesByPrefixParams - Mark ResourceType Enum as FLAG

## Feature request for Cloudinary .NET SDK
Mark resourceType enum with flag so you could specify both images and videos


## Explain your use case
Currently you have to do something like this to retrieve both images and photos:

```
var @params1 = new ListResourcesByPrefixParams
{
ResourceType = ResourceType.Video,
Type = "upload",
Prefix = "SomeFolder",
};

var resources1 = await _cloudinary.ListResourcesAsync(@params1);

var @params2 = new ListResourcesByPrefixParams
{
ResourceType = ResourceType.Image,
Type = "upload",
Prefix = "SomeFolder",
};

var resources2 = await _cloudinary.ListResourcesAsync(@params1);

var resources = resources1.Resources.Concat(resources2.Resources);
```