[dsrouter] Implement a WebSocket server

[lambdageek]

In .NET 8, the .NET runtime may ship a variant of the WebAssembly runtime that supports diagnostic tracing (this is an optional opt-in because it requires threading support, which is opt-in). Runtime tracking issue: dotnet/runtime#69268

In order to support tracing in WebAssembly apps, the dotnet-dsrouter tool needs a new mode where it can create a WebSocket server that will allow a .NET app running in the browser to connect to it.

$ dotnet-dsrouter server-websocket -ipcs ./domain_socket -ws ws://localhost:12345/diagnostics

After that, the usual diagnostics commands such as dotnet-trace will be able to collect diagnostics from running WebAssembly apps.

$ dotnet-trace collect --diagnostic-port ./domain_socket,connect ...