Support starting DTD and setting workspace roots from integration tests

This is a follow up to https://github.com/flutter/devtools/issues/9176. To emulate the runtime environment where DevTools server was started with a DTD uri passed to it on start, we need to be able to start DTD from integration tests before we start the DevTools server.

This will unblock use cases like testing static DevTools extensions that should be present without a connected app, based on the roots set by the DTD instance that was started by the user's IDE.

To do this, we can re-use this code: https://github.com/dart-lang/sdk/blob/main/pkg/dds/lib/src/devtools/dtd.dart. We may want to expose this so we can import it directly from `devtools_app`.

EDIT: it may be better to run `dart tooling-daemon` directly.

首先检查集成测试设置以及 Dart SDK 中 pkg/dds/lib/src/devtools/dtd.dart 的现有 DTD 实现。了解 devtools_app 如何在 DevTools 服务器启动前提供或调用此支持，并将该方法与直接运行 dart tooling-daemon 进行比较。当集成测试能够在 DevTools 之前启动 DTD，并为静态扩展场景设置工作区根目录时，即视为完成。

由索引模型根据 Issue 内容生成。