Support custom data sources

I want to use image classification, but I have images in heterogeneous locations, and some need some kind of custom pre-processing, so the current image data sources is very limiting

Also, there's some other scenarios I'm facing which might require the images to feed to ML will need some sort of preprocessing. It's a big waste to have to write them to temporary files.

```json
"DataSource": {
"Type": "Folder",
"Version": 1,
"FolderPath": "c:\\Images"
},
```

But I would like to do something like this:

```json
"DataSource": {
"Type": "Class",
"Version": 1,
"ClassName": "CustomImageImporter"
},
```
```c#
class CustomImageImporter : IImageClassifierImporter
{
public IEnumerable<(string Label, Byte[] Image)> EnumerateLabeledInputImages()
{
yield return ("cat", LoadBytes("cat.png"));
yield return ("dog", LoadBytes("dog.png"));
}
}
```

The only alternative I have right now is not using AutoML and writing the whole thing in ML