LSP Client Initialize should return after capabilities response and not wait until server Initialized Message

The Debug Adapter Protocol spec states:

Initialization happens first, and the debug adapter must respond to the initialize request with any capabilities before any further communication can take place.
At any point after the client receives the capabilities, it sends a launch or attach request.
Once the debug adapter is ready to receive configuration from the client, it sends an initialized event to the client. As described above, the client sends zero or more configuration-related requests before sending a configurationDone request.

It is valid for the client to send launch/attach requests prior to initialized being received from the server, and this is required for some DAP extensions (PowerShell), but without getting the client adapter back from the await, this is currently not possible with the client.

Workaround

Add a OnInitialized handler that completes a TaskCompletionSource with the client early, and don't wait on the From or InitializeClient Tasks.

Start by reading the DAP initialization flow, especially the From and InitializeClient tasks and the OnInitialized handler described in the workaround. Compare when capabilities are received with when the client becomes available; done means launch or attach can be sent before the server's initialized event without waiting for that event.