all: run gofumpt (#652)

* all: run gofumpt

https://github.com/mvdan/gofumpt

The main appeal for me here is that my (and many other peoples')
editors run this on save, so doing a single diff here
keeps other diffs minimal.

* .github/workflows: add gofumpt checker

libplugin/ioconn/conn_test.go

@@ -17,7 +17,6 @@ func TestDial(t *testing.T) {
 	defer conn.Close()
 
 	go func() {
-
 		_, _ = conn.Write([]byte("hello"))
 	}()
 	buf := make([]byte, 5)

libplugin/pluginbase.go

@@ -164,7 +164,6 @@ func (s *server) Logs(req *StartLogRequest, stream SshPiperPlugin_LogsServer) er
 }
 
 func (s *server) ListCallbacks(ctx context.Context, req *ListCallbackRequest) (*ListCallbackResponse, error) {
-
 	var cb []string
 
 	if s.config.NewConnectionCallback != nil {
@@ -362,7 +361,6 @@ func (s *server) KeyboardInteractiveAuth(stream SshPiperPlugin_KeyboardInteracti
 
 		return userInput.Answers[0], nil
 	})
-
 	if err != nil {
 		return err
 	}