[flutter_tools/dap] Add support for forwarding `flutter run --machine` exposeUrl requests to the DAP client (#114539)

* [flutter_tools/dap] Add support for forwarding `flutter run --machine` requests to the DAP client

Currently the only request that Flutter sends to the client is `app.exposeUrl` though most of this code is generic to support other requests that may be added in future.

* Improve comment

* Fix thrown strings

* StateError -> DebugAdapterException

* Add a non-null assertion and assert

* Use DebugAdapterException to handle restartRequests sent before process starts

* Fix typo + use local var

* Don't try to actually send Flutter messages in tests because there's no process

[flutter_tools/dap] Add a base Flutter adapter class to avoid duplication between adapters (#114533)

[dap] Don't wait for appStarted before responding to launch/attach + don't call app.stop for unstarted app (#109386)

* [flutter_tools] [dap] Ensure DAP sends app.stop/app.detach during terminate

Fixes an issue where the flutter_tester device may not be cleaned up correctly if we just terminate the Flutter process.

* Update integration test expectations

* Revert accidental commit

* [flutter_tools] [dap] Add support for passing env variables to spawned processes

* Use named args

* Use in-memory fs and FakePlatform

* Pass filesystem style to MemoryFileSystem