Support the new Variant API in AGP 7.0+

## What feature would you like to see?

Currently, App Distribution gradle plugin provides the `AppDistributionExtension` extension on `buildType` and `productFlavor`

Feature request:

In the new Variant API in AGP 7.0+ plugins need to register their extensions using [`AndroidComponentsExtension.registerExtension`](https://developer.android.com/reference/tools/gradle-api/7.0/com/android/build/api/variant/AndroidComponentsExtension#registerExtension(com.android.build.api.variant.DslExtension,%20kotlin.Function1))

`AppDistributionExtension` needs to implement `VariantExtension`

Then it needs something like
```
androidComponents {
registerExtension(...
}
```

[See this example code.
](https://github.com/android/gradle-recipes/blob/8f50ef8e861ee4a41c2f92dda3992d40fa0e444f/BuildSrc/customizeAgpDslAndVariant/buildSrc/src/main/kotlin/ProviderPlugin.kt#L46)

## How would you use it?

Would configure App Distribution using the new AGP variant API.