dtls-in-stun: Resend last part of handshake when becoming ICE writable.

When receiving the STUN_BINDING_RESPONSE on a dtls-client with piggy
backing, the DTLS attribute is processed before ICE becomes writable.
The DTLS attribut makes DTLS writable...but a last packet needs to be
transported or else the server can not receive packets from the client.

This patch makes sure that the last packet is sent.

Test improvments:
- Add FakeIceTransport feature to drop non stun packets unless writable.
- Add count of "DTLS retransmission" (maybe also add to stats?)
- Add count of DTLS data received by STUN.

Bonus: Found Connection.cc propagating DTLS-in-stun attributes
*before* the STUN_BINDING_RESPONSE was authenticated. The same
attributes were already propagated in the correct place (
Connection::OnConnectionRequestResponse) after the RESPONSE
was fully verified. This part lacks unit tests :( But is
tested in the integration test using the new counter I added.

BUG=webrtc:367395350

Change-Id: I126abdcb062d95389fd5ce1376c212a3634f07a6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/382981
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44229}