Expose RequestBuilder's model

**Glide Version**: 4.15.1

**Issue details / Repro steps / Use case background**:
Can Glide expose a `RequestBuilder`'s model? I'm building a composable that can take glide requests, but it's proving hard to find a key for remembering requests across compositions.

```kotlin
val request = Glide.with(context)
.load("https://example.com/image.jpg")
.thumbnail(…)
.transition(…)

val downloader = remember(request.model) {
GlideImageDownloader(request)
}
```

Using `request` as the remember key isn't ideal because `RequestBuilder` is not a stable API. For example, `withCrossFade(100) == withCrossFade(100)` will always be false.

Start with the RequestBuilder API and trace how the model supplied through Glide.with(...).load(...) is retained. Define a stable way for callers such as GlideImageDownloader to access that model and verify that it can be used as a composition key without relying on RequestBuilder equality.

Written by the indexing model from the issue text.