Add support for Echo requests

[jgiannuzzi]

This PR adds support for Echo requests/responses, which can be used as an application layer keepalive.

[jgiannuzzi]

Hi @arashpayan, could you please review this PR?

For context, I'm working on improving the rclone smb backend. One of the things I'd like to do is using Echo as a way to reduce the overhead when testing that connections are still working.

[arashpayan]

Thanks for the PR @jgiannuzzi. Looks good and it works against my Samba instance. I just have one request. Could you add a test to the smb2_test.go file? Something like:

func TestEcho(t *testing.T) {
	if session == nil {
		t.Skip()
	}

	err := session.Echo()
	require.NoError(t, err)
}

As a bonus, when you save smb2_test.go, your formatter will probably update all the octal values to the new format.

[arashpayan]

To clarify, I'm fine with the updated octal format.

[jgiannuzzi]

Good point about the tests – I didn't realise how they were setup! That's now added.
My formatter is gofumpt but it's not configured with the -extra flag because this is not yet supported within gopls (see mvdan/gofumpt#251) – the result is that it didn't change the octal values!

[arashpayan]

Thanks again for the contribution @jgiannuzzi.