feat: add --header-command flag (#9059)

This allows specifying a command to run that can output headers for
cases where users require dynamic headers (like to authenticate to their
VPN).

The primary use case is to add this flag in SSH configs created by the
VS Code plugin, although maybe config-ssh should do the same.

cli/login.go

@@ -76,7 +76,7 @@ func (r *RootCmd) login() *clibase.Cmd {
 				serverURL.Scheme = "https"
 			}
 
-			client, err := r.createUnauthenticatedClient(serverURL)
+			client, err := r.createUnauthenticatedClient(ctx, serverURL)
 			if err != nil {
 				return err
 			}