[native_assets_cli] Reorganize extension schemas

[dcharkes]

Closes: #2085

This PR makes the code asset and data asset extension shared_definitions.schema.json only specify the schemas of the extensions. Not where they are applied in the base schema.

These extension definitions are then used to generate the syntax for the extensions, which means the syntax of the extensions no longer refers to the base protocol syntax. (The imports have disappeared.)

To document how the extension definitions extend the base protocol, this PR adds a code generator that generates the repetitive structure for combining the base protocol with the extension.

We're keeping these combined base + extension schemas because they are the best documentation for where the extensions fit on the base protocol. Also the schema tests keep using these.

[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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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 86.663% (-0.09%) from 86.751%
when pulling bae5f11 on subschemas
into 0cd2e55 on main.

[dcharkes]

@jonasfj We decided to restructure slightly how extensions are modeled. PTAL.