[Dart] Send dart/connectToDtd request to the Analysis Server

[elliette]

Fixes flutter/flutter-intellij#7669

dart/connectToDtd request documented here: https://github.com/dart-lang/sdk/blob/bc9def5da7804243a9cce531e3512ab94a569fcd/pkg/analysis_server/tool/lsp_spec/README.md?plain=1#L280

[elliette]

FYI @alexander-doroshko @jwren

[alexander-doroshko]

According to the spec, dart/connectToDtd is a request that the IDE should send to the Dart Analysis Server. This PR tries to send it to the Dart Tooling Daemon. I guess DTD doesn't need to connect to DTD :).

The scope of this subtask is to implement DartAnalysisServerService.connectToDtd(String url) method. It will be similar to lspMessage_dart_textDocumentContent() and it will require DAS protocol files update (com.google.dart.server.generated.AnalysisServer and friends). Some of these files could be generated, others - hand-written. I think @jwren can help with it.