consolidate AssetBundle::entries and AssetBundle::entryKinds into a new type, `AssetBundleEntry` (#142029)

Part of work on https://github.com/flutter/flutter/pull/141194

The [`AssetBundle`](https://github.com/flutter/flutter/blob/0833929c997c8a9db11c1e0df9a731ab17b77606/packages/flutter_tools/lib/src/asset.dart#L80) class contains two members, `entries` and `entryKinds`. `entries` contains asset data indexed by asset key. `entryKinds` contains the "kinds" of these assets, again indexed by asset key.

**Change.** Rather than have two separate maps, this PR proposes combining these maps into one by wrapping the asset data and kind into a single data type `AssetBundleEntry`.

**Purpose.** In https://github.com/flutter/flutter/pull/141194, I am considering associating more information with an asset. In particular, what transformers are meant to be applied to it when copying it to the build output. Rather than adding another map member onto `AssetBundle` (e.g. `entryTransformers`), I decided to make things neater by introducing the `AssetBundleEntry` type.