[native_assets_cli] Cleanup TODOs

[dcharkes]

• We now have user-defines, so use them in the example for downloading assets.
• Remove todos that we decided we're not going to address.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
native_assets_builder	Breaking	0.16.0	0.17.0-wip	0.17.0	✔️
native_assets_cli	Breaking	0.16.0	0.17.0-wip	0.17.0	✔️

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// 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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

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

[coveralls]

coverage: 77.204%. remained the same
when pulling a55bc23 on cleanup-todos
into 4111cde on main.

[goderbauer]

LGTM