[native_assets_cli] Syntax for assets_for_build and hooks/metadata

[dcharkes]

Bug: #1251

This PR contains the JSON spec for:

• [native_assets_cli] Unify Metadata with Assets #2164

For more details see that PR.

This PR adds the syntax for passing assets from build hooks to build hooks.

build output assets_for_build:

{
  "assets": [],
  "assets_for_build": [
    {
      "encoding": {
        "a_key": "some_value"
      },
      "some_key": "some_value",
      "type": "some_asset_type"
    }
  ],
  "assets_for_linking": {},
}

build input assets per package name:

{
  "assets": {
    "some_package": [
      {
        "some_key": "some_value",
        "type": "some_asset_type"
      }
    ]
  },
  "config": {},
}

Secondly, this PR adds a syntax for metadata assets (assets that cannot be bundled but just ferry information between different hooks).

      {
        "encoding": {
          "key": "foo",
          "value": "bar"
        },
        "type": "hooks/metadata"
      },

The schemas now check these.

And this is covered by tests.

Changes to the generated schemas:

• Override the new build input asset and build output assets_for_build with the CodeAsset and DataAsset in the generated extension schemas.

Notable changes to the syntax generator:

• Enable generating tagged enum classes if there are two fields (a tag, and some encoding) in the super class.
• Support Object fields (used for MetadataAsset.value).

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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[coveralls]

coverage: 84.822% (-0.3%) from 85.171%
when pulling cf34082 on metadata-refactor-syntax
into 18e2518 on main.