[ffigen] Split protocol building stuff into a $Builder class

[liamappelbe]

Split protocol building stuff into a $Builder class and generate bindings for the protocol's instance methods. This allows the user to invoke methods on the protocol instance.

Fixes #2492

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.1.0	9.0.0-dev.0	9.0.0	✔️

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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 symbol	Leaking sources
objective_c	_FinalizablePointer	internal.dart::_ObjCReference::new::_finalizable

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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.

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no missing headers

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Unrelated files missing license headers

Files
pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 92.256% (+0.3%) from 91.972%
when pulling 14ccdcd on ffigen_proto_inst
into f7c7d52 on main.