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

ソースファイルやテストは指定されていません。まず、Type "Folder" の JSON DataSource がどのように処理されているか、また IImageClassifierImporter をどこに組み込めそうかを追跡してください。カスタム importer が、異なる場所にあるラベル付き画像バイト列を、テンポラリファイルを必要とせずに前処理付きで提供できれば完了です。

索引モデルが issue の本文から書いたものです。