Explore API for injecting a middle-man into DAP communication

[weinand]

Provide API to "tap" on the DAP message flow so that LiveShare no longer needs to wrap or proxy DAs.

[weinand]

New proposed API:

https://github.com/Microsoft/vscode/blob/88b85b66c3abc225d09696a0943aa652a83cfdc5/src/vs/vscode.proposed.d.ts#L560-L567

Example: tracing DAP for all debug sessions:

vscode.debug.registerDebugConfigurationProvider('*', {
  provideDebugAdapterTracker(session: vscode.DebugSession, folder: WorkspaceFolder | undefined, config: DebugConfiguration, token?: CancellationToken): ProviderResult<vscode.IDebugAdapterTracker> {
    return {
      startDebugAdapter: () => console.log(`startDebugAdapter: ${session.id}`),
      fromDebugAdapter: m => console.log(`<=== ${JSON.stringify(m, undefined, 2)}`),
      toDebugAdapter: m => console.log(`===> ${JSON.stringify(m, undefined, 2)}`),
      debugAdapterError: err => console.log(`debugAdapterError: ${err}`),
      debugAdapterExit: (code, signal) => console.log(`debugAdapterExit: ${code}`),
      stopDebugAdapter: () => console.log(`stopDebugAdapter: ${session.id}`)
    };
  }
});