[TLS] [DTLS] ACK's for post-handshake authentication requests

[Hanno Becker <[e-mail]>]

I have a minor comment on DTLS 1.3 draft 37.

On the topic of sending ACKs, the draft recommends:

```
ACKs SHOULD NOT be sent for other complete
flights because they are implicitly acknowledged by the receipt of
the next flight, which generally immediately follows the flight.
```

I wonder if the case of post-handshake authentication should be
explicitly mentioned as a potential exception to this rule, since
the TLS 1.3 RFC explicitly mentions that responses to
`CertificateRequest` may be delayed in some contexts:

```
Note: Because client authentication could involve prompting the user,
servers MUST be prepared for some delay, including receiving an
arbitrary number of other messages between sending the
CertificateRequest and receiving a response.
```

In this case, it would be beneficial to immediately explicitly ACK
the `CertificateRequest` message even though it is also implicitly
acked through the eventual response via the `Certificate` message.

Regards,
Hanno
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